# Vote splitting

(Redirected from Spoiler effect)

Vote splitting is an electoral effect in which the distribution of votes among multiple similar candidates reduces the chance of winning for any of the similar candidates, and increases the chance of winning for a dissimilar candidate. This is commonly known as the spoiler effect, which can discourage minor party candidacies.

Vote splitting most easily occurs in plurality voting (also called first-past-the-post) in which each voter indicates a single choice and the candidate with the most votes wins, even if the winner does not have majority support.[1] For example, if candidate A1 receives 30% of the votes, similar candidate A2 receives another 30% of the votes, and dissimilar candidate B receives the remaining 40% of the votes, plurality voting declares candidate B as the winner, even though 60% of the voters prefer either candidate A1 or A2.

## Spoiler effect

The spoiler effect is the effect of vote splitting between candidates or ballot questions[n 1] who often have similar ideologies. One spoiler candidate's presence in the election draws votes from a major candidate with similar politics, thereby causing a strong opponent of both or several to win.[2][3][4][5] The minor candidate causing this effect is referred to as a spoiler.[n 2]

The problem also exists in two-round system and instant-runoff voting[4][6][7][8][9][10] though it is reduced, because weaker spoilers are eliminated. However, a candidate that can win head-to-head against all rivals (Condorcet winner) can still lose from third place in a 3-way vote split, a phenomenon known as the center squeeze. This occurred in the 2009 Burlington Vermont mayoral election and the 2022 Alaska's at-large congressional district special election. Some ranked voting systems satisfy the Condorcet winner criterion. Other preferential voting systems also suffer from variations of the spoiler effect, as they fail the independence of irrelevant alternatives (IIA) criterion (see § Mathematical definitions).

Some argue the problem is reduced in cardinal voting methods like approval voting, score voting, or majority judgment, since the rating of each candidate is independent of the ratings of other candidates, however this argument requires that some voters having meaningful preferences in an election with only two alternatives necessarily casting votes which has little or no voting power, or necessarily abstaining. If it is assumed to be at least possible that any voter having preferences might not abstain, or vote their favorite and least favorite candidates at the top and bottom ratings respectively, then these systems may not immune to vote splitting. It can be claimed that cardinal ballots themselves are immune to vote splitting (i.e. altering the ballot after they have been cast), but not the internal voter preferences (i.e. changing the context in which the ballots were created).[11][12] Cardinal voting methods also fail the independence of irrelevant alternatives criterion.

### Relationship with other effects

The spoiler candidate may alter an election result if they cause voters to switch their votes away from those seen as more electable and thus more politically viable[n 3], a common effect called vote splitting. If one opposing candidate is ideologically or politically similar and therefore receives far fewer votes than other opposing candidates to the spoiler candidate, vote splitting has a spoiler effect.

In some cases, even though spoiler candidates cannot win themselves, their influence upon the voters may enable the candidate to determine deliberately the more viable candidate who wins the election, a situation known as a kingmaker scenario. With a first-past-the-post voting system, that is particularly feasible when spoiler candidates recommend tactical voting or run on a false manifesto to bolster the prospects for another candidate to win.

In a preferential voting system, voters can feel more inclined to vote for a minor party or independent as their first choice and can record a preference between the remaining candidates, whether they are in a major or established party or not. For example, voters for a minor left-wing candidate might select a major left-wing candidate as their second choice, thus minimizing the probability that their vote will result in the election of a right-wing candidate, or voters for an independent candidate perceived as libertarian, or simply as the voter prefers that ideology might select a particular libertarian candidate as their second choice, thus minimizing the probability of an authoritarian candidate being elected. Approval voting and proportional representation systems can also reduce the spoiler effect.

One of the main functions of political parties is to mitigate the effect of spoiler-prone voting methods by winnowing on a local level the contenders before the election. Each party nominates at most one candidate per office since each party expects to lose if they nominate more than one.[n 4] In some cases, a party can expect to "lose" by "suffering a rival elected opponent" if they nominate more than zero, where two opponents exist and one is considered a candidate they can "work with" — a party may prefer the candidate who would win if the party nominates zero.[n 5]

Thus, empirical observations of the frequency of spoiled elections do not provide a good measure of how prone to spoiling a particular voting method is, since the observations omit the relevant information about potential candidates who did not run because of not wanting to spoil the election.[original research?]

### Mathematical definitions

Possible mathematical definitions for the spoiler effect include failure of the independence of irrelevant alternatives (IIA) axiom, and vote splitting.

Arrow's impossibility theorem states that rank-voting systems are unable to satisfy the independence of irrelevant alternatives criterion without exhibiting other undesirable properties as a consequence. Gibbard's theorem shows that the same is true of cardinal voting systems like approval and score voting. However, different voting systems are affected to a greater or lesser extent by IIA failure. For example, instant runoff voting is considered to have less frequent IIA failure than First Past the Post (also known as Plurality Rule). The independence of Smith-dominated alternatives (ISDA) criterion is much weaker than IIA; unlike IIA, some ranked-ballot voting methods can pass ISDA.

A possible definition of spoiling based on vote splitting is as follows: Let W denote the candidate who wins the election, and let X and S denote two other candidates. If X would have won had S not been one of the nominees, and if (most of) the voters who prefer S over W also prefer X over W (either S>X>W or X>S>W), then S is a spoiler. Here is an example to illustrate: Suppose the voters' orders of preference are as follows:

 33%: S>X>W 15%: X>S>W 17%: X>W>S 35%: W>X>S

The voters who prefer S over W also prefer X over W. W is the winner under Plurality Rule, Top Two Runoff, and Instant Runoff. If S is deleted from the votes (so that the 33% who ranked S on top now rank X on top) then X would be the winner (by 65% landslide majority). Thus S is a spoiler with these three voting methods.

## By electoral system

Different electoral systems have different levels of vulnerability to vote splitting. All voting systems have some degree of inherent susceptibility to strategic nomination considerations. Strategic nomination takes advantage of vote splitting to defeat a popular candidate by supporting another similar candidate. Vote splitting is one possible cause for an electoral system failing the independence of clones or independence of irrelevant alternatives fairness criteria.

Plurality-runoff voting methods (like exhaustive ballot, two-round system/top-two primary,[1] instant-runoff voting,[13] supplementary vote, and contingent vote) still suffer from vote-splitting in each round, but can somewhat reduce its effects compared to single-round plurality voting.[14]

Cardinal voting methods are immune to vote splitting if it is assumed that voters rate candidates individually and independently of knowing the available alternatives in the election, using their own absolute scale, since each candidate is rated independently of each other.[13] This assumption implies that some voters having meaningful preferences in an election with only two alternatives will necessarily cast a vote which has little or no voting power, or necessarily abstain. If it is assumed to be at least possible that any voter having preferences might not abstain, or vote their favorite and least favorite candidates at the top and bottom ratings respectively, then these systems are not immune to vote splitting.

Vote pairing (also called vote swapping, co-voting or peer to peer voting) can mitigate the effect, but it requires two voters in different districts to agree, and identifying probabilities of candidates winning in those districts. A vote swap effectively preserves the total support for each party but moved it to where it is most effective. It is legal and practiced in US,[15] Canadian[16] and especially UK elections,[17] as well as in some Australian state elections. Pairwise-counting Condorcet methods minimize vote splitting effects.[14][1]

### Plurality voting

Vote splitting most easily occurs in plurality voting because the ballots gather only the least bad preference of the voter.[18] In the United States vote splitting commonly occurs in primary elections.[14] The purpose of primary elections is to eliminate vote splitting among candidates in the same party before the general election. If primary elections or party nominations are not used to identify a single candidate from each party, the party that has more candidates is more likely to lose because of vote splitting among the candidates from the same party. Primary elections occur only within each party and so vote splitting can still occur between parties in the secondary election. In open primaries, vote splitting occurs between all candidates.

In addition to applying to single-winner voting systems (such as used in the United Kingdom, the United States and Canada), a split vote can occur in proportional representation methods that use election thresholds, such as in Germany, New Zealand and Turkey. In those cases, "fringe" parties that do not meet the threshold can take away votes from larger[clarification needed] parties with similar ideologies.

### Ordinal voting methods

When ranked ballots are used, voters can vote for a minor party candidate as their first choice and indicate their order of preference for the remaining candidates, without regard to whether a candidate is in a major political party. For example, voters who support a very conservative candidate can select a somewhat-conservative candidate as their second choice, thus minimizing the chance that their vote will result in the election of a liberal candidate.

Runoff voting is less vulnerable to vote splitting than is plurality voting, but vote splitting can occur in any round of runoff voting.

Vote splitting rarely occurs when the chosen electoral system uses ranked ballots and a pairwise-counting method, such as a Condorcet method.[14] Pairwise counting methods do not involve distributing each voter's vote between the candidates. Instead, pairwise counting methods separately consider each possible pair of candidates for all possible pairs. For each pair of candidates, there is a count for how many voters prefer the first candidate (in the pair) to the second candidate and how many voters have the opposite preference. The resulting table of pairwise counts eliminates the step-by-step distribution of votes, which facilitates vote splitting in other voting methods.

Voting methods that are vulnerable to strategic nomination, especially those that fail independence of clones, are vulnerable to vote splitting. Vote splitting also can occur in situations that do not involve strategic nomination, such as talent contests (such as American Idol) in which earlier rounds of voting determine the current contestants.

### Cardinal voting methods

Cardinal voting methods require an independent score to be given to candidates, as opposed to a ranking. The three primary methods are approval voting, with a range between 0–1, score voting with an arbitrary range, and STAR voting.

Cardinal voting methods are immune to vote splitting if it is assumed that voters rate candidates individually and independently of knowing the available alternatives in the election, using their own absolute scale, since each candidate is rated independently of each other.[13] This assumption implies that some voters having meaningful preferences in an election with only two alternatives will necessarily cast a vote which has little or no voting power, or necessarily abstain. If it is assumed to be at least possible that any voter having preferences might not abstain, or vote their favorite and least favorite candidates at the top and bottom ratings respectively, then these systems are not immune to vote splitting.

An alternative interpretation for the cardinal case is that the ballots themselves are immune to vote splitting (i.e. altering the ballot after they have been cast), but not the internal voter preferences (i.e. changing the context in which the ballots were created).[19][20]

## Election examples by country

### Australia

In Australia, seats where vote splitting occurs are called "three-corned contests". While the vote is split in a three-cornered contest, it is not always a disadvantage as Australia uses preferential voting. However, depending on the level of government it can still act as a disadvantage due to the different forms of preferential voting used in Australia; full preferential voting (FPV) is used on a federal level and in some states and territories while optional preferential voting (OPV) is used in New South Wales. Due to this, three-cornered contests are rare in New South Wales (on both a state and federal level) as well as in the federal Senate.

Three-cornered contests generally occur with the centre-right Liberal-National Coalition. However, the frequency of these contests varies in different states and it does not occur in the Queensland, Tasmania or the two territories, due to the fact that the Nationals do not exist in the Australian Capital Territory (ACT) and they do not currently contest elections in Tasmania, while in Queensland and the Northern Territory the two Coalition parties merged to become the Liberal National Party (LNP) and the CLP, respectively. While they are rare in New South Wales, three-cornered contests do often occur in Victoria, Western Australia and South Australia.

#### Federal politics

Federally, three-cornered contests are uncommon in most seats. However, they do occur in certain regional seats. At the 2022 federal election, both Coalition parties ran candidates in four seats; two in Victoria (Division of Indi, Nicholls), one in Western Australia (Durack) and one in South Australia (Barker). The number of federal seats with three-cornered contests has dropped over the years. In fact, there was a significantly low number of three-cornered contests in 2022, even when compared to the previous federal election, which was held in 2019. In that election, both Coalition parties ran candidates in ten seats; two in New South Wales (Eden-Monaro and Gilmore), two in Victoria (Indi and Mallee), two in Western Australia (Durack and O'Connor), one in South Australia (Barker) and three in Tasmania (Bass, Braddon and Lyons).

#### State politics

In New South Wales (the only state where the Coalition has never been broken), three-cornered contests are rare as most Coalition voters exhaust their preferences (meaning they only number one candidate, thus being a disadvantage except in conservative strongholds on the Mid North Coast and in central and western parts of the state). However, at the 2023 state election, this phenomenon did occur in two regional seats: Port Macquarie (a conservative seat held by National-turned-Liberal MP Leslie Williams) and Wagga Wagga (a traditionally conservative seat, despite being held by independent MP Joe McGirr). In Port Macquarie, Williams was the Liberal candidate and Peta Pinson, the Mayor of the Port Macquarie-Hastings Council, was the Nationals candidate. Williams won the seat, which despite having a three-cornered contest remained a safe seat on both two-candidate-preferred (TCP) and two-party-preferred (TPP) margin. In Wagga Wagga, the Nationals candidate was Adrianna Benjamin and the Liberal candidate was Julia Ham. McGirr retained the seat with an increased TCP margin against Benjamin, although the Nationals still won the TPP count. It is unlikely that a three-cornered contest will occur in either of these seats at the next state election, which will be held in 2027.

In Victoria, three-cornered contests occur in some seats, despite the Coalition existing in Victoria (although it has previously been broken). At the 2022 state election, both Coalition parties ran candidates in five seats (Bass, Euroa, Mildura, Morwell and Shepparton). The Nationals intended to run a candidate in the Narracan, but the candidate they preselected died before the election, forcing a supplementary by-election in that seat, in which the Liberal candidate was re-elected and the Nationals did not run a candidate.

In Western Australia, three-cornered contests do commonly occur. This is due to the fact that the Coalition agreement is different in that both parties are independent of each other and each party can vote differently if they believe that their decision it is in the best interests of the people and areas they represent. The Nationals can also opt-out of Cabinet and when a Coalition government is elected, the leader of the Liberal Party becomes the state Premier, but the leader of the Nationals does not always become the Deputy Premier, unlike in New South Wales and Victoria where in the event of a Coalition government, the Liberal leader becomes the Premier and the Nationals leader becomes the Deputy Premier. For example, following the 2008 state election, which saw a Coalition government elected, the Liberal leader (Colin Barnett) became the Premier, but the Nationals leader (Brendon Grylls) did not become the Deputy Premier, an office that the deputy Liberal leader (Kim Hames) was given instead.

In South Australia, three-cornered-contests do occur in some seats, due to the absence of the Coalition. In most states, the Liberal Party holds seats in cities while the Nationals hold seats in regional, rural and remote areas, but in South Australia and Tasmania, the Nationals have limited activity and thus the Liberals hold both metropolitan and non-metropolitan seats in these states. At the 2022 state election, both the Liberals and the Nationals ran candidates in eight seats (Chaffey, Finniss, Flinders, Frome, Hammond, MacKillop, Narungga and Schubert). Due to the limited activity of the party, the Nationals finished last or close-to-last in all of these seats, even being outvoted by some minor parties and winning a statewide vote of just 0.48%, the lowest in the country on a state level (excluding states the party does not contest elections in).

In Australia, the 1918 Swan by-election saw the conservative vote split between the Country Party and Nationalist Party, which allowed the Australian Labor Party to win the seat. That led the Nationalist government to implement preferential voting in federal elections to allow Country and Nationalist voters to transfer preferences to the other party and to avoid vote splitting.[21] Today, the Liberal Party and National Party rarely run candidates in the same seats, which are known as three-cornered contests. When three-cornered contests do occur the Labor Party would usually direct preferences to the Liberals ahead of the Nationals as they considered the Liberal Party to be less conservative than the Nationals. The 1996 Southern Highlands state by-election in New South Wales is an example of this when the Nationals candidate Katrina Hodgkinson won the primary vote but was defeated after preferences to Liberal candidate Peta Seaton when Seaton received Labor Party preferences.[22][23][24]

### Bosnia and Herzegovina

In 2006, the HDZ 1990 broke away from the HDZ BiH this allowed Željko Komšić to gain the Croat membership in the Presidency of Bosnia and Herzegovina with less than 40% which mainly came from Bosniak areas.[citation needed]

When the cities of Fort William and Port Arthur merged and (in 1969) voted on a name for the new town, the vote was split between the popular choices of "Lakehead" and "The Lakehead", allowing the third option to win, creating the town of Thunder Bay, Ontario.[25]

From 1993 to 2004, the conservative vote in Canada was split between the Progressive Conservatives and the Reform (later the Alliance) Party. That allowed the Liberal Party to win almost all seats in Ontario and to win three successive majority governments.

The 2015 provincial election in Alberta saw the left-wing New Democratic Party win 62% of the seats with 40.6% of the province's popular vote after a division within the right-wing Progressive Conservative Party, which left it with only 27.8% of the vote, and its breakaway movement, the Wildrose Party, with 24.2% of the vote. In 2008, the last election in which the Progressive Conservative Party had been unified, it won 52.72% of the popular vote. The Progressive Conservatives had won every provincial election since the 1971 election, making them the longest-serving provincial government in Canadian history—being in office for 44 years. This was only the fourth change of government in Alberta since Alberta became a province in 1905, and one of the worst defeats a provincial government has suffered in Canada. It also marked the first time in almost 80 years that a left-of-centre political party had formed government in Alberta since the defeat of the United Farmers of Alberta in 1935 and the Depression-era radical monetary reform policies of William Aberhart's Social Credit government. During the 2021 Canadian federal election, it is speculated that the People's Party of Canada might have coast the CPC up to 24 seats.[26]

In Canada, vote splits between the two major left-of-centre parties (Liberals and NDP) assisted the Conservative Party in winning the 2006, 2008, and 2011 federal elections, despite most of the popular vote going to left-wing parties in each race. During the 2022 Ontario General Election, Progressive Conservative Doug Ford won a second term as Premier of the Province of Ontario. The Progressive Conservatives won several ridings due to vote splitting.[27] ONDP and Liberal Party voters combined for 47.8% of votes, whereas Ford emerged victorious with only 40.82% of total votes.[28]

Similarly, in Quebec, it is argued that the success of the Bloc Québécois in elections from 1993 to 2008 was because of the federalist vote being split between the Liberals and the Conservatives.[citation needed]

### Egypt

In the 2012 Egyptian presidential election, the two candidates who qualified for the runoff election, Freedom and Justice Party candidate Mohamed Morsi (24.8%) and the independent candidate Ahmed Shafik (23.7%), each received more votes than any other candidate, but they failed to get enough votes to prove that each winning candidate was actually more popular than the Dignity Party candidate Hamdeen Sabahi (20.7%), the independent candidate Abdel Moneim Aboul Fotouh (17.5%), or the independent candidate Amr Moussa (11.1%).[citation needed]

### France

In France, the 2002 presidential elections have been cited as a case of the spoiler effect: the numerous left-wing candidates, such as Christiane Taubira and Jean-Pierre Chevènement, both from political parties allied to the French Socialist Party, or the three candidates from Trotskyist parties, which altogether totalled around 20%, have been charged with making Lionel Jospin, the Socialist Party candidate, lose the two-round election in the first round to the benefit of Jean-Marie Le Pen, who was separated from Jospin by only 0.68%. Some also cite the case of some districts in which the moderate right and the far right had more than half of the votes together, but the left still won the election; they accuse the left of profiting from the split. Also in the presidential elections 1969 (with five left-wing candidates which combined had 32%), in 2017 (split between four candidates which had 27% combined) and in 2022 (six left-wing candidates with 32% combined), the left failed to reach the run-off which may be traced back the amount of left-of-centre candidates. Similarly in the 1993 parliamentary election, where the green parties ran against the parties of the presidential majority. This led to many right-wing run-offs and the most right-wing dominated parliament since 1968.

In the 2023 French Polynesian legislative election, the anti-separatist A here ia Porinetia did not form an alliance with the Tāpura Huiraʻatira allowing the separatist Tāvini Huiraʻatira to win the run-off with just 44%.[citation needed]

### Germany

In the German presidential election of 1925, Communist Ernst Thälmann refused to withdraw his candidacy although it was extremely unlikely that he would have won, and the leadership of the Communist International urged him not to run. In the second (and final) round of balloting, Thälmann shared 1,931,151 votes (6.4%). Centre Party candidate Wilhelm Marx, backed by pro-republican parties, won 13,751,605 (45.3%). The right-wing candidate Paul von Hindenburg won 14,655,641 votes (48.3%).[29] If most of Thälmann's supporters had voted for Marx, Marx likely would have won the election. That election had great significance because after 1930, Hindenburg increasingly favoured authoritarian means of government, and in 1933, he appointed Adolf Hitler as chancellor. Hindenburg's death the following year gave Hitler unchecked control of the German government.[30]

Klimaliste has been accused of splitting the vote which would have gone to Alliance 90/The Greens.[31] For example, in the 2021 Baden-Württemberg state election a Red-Green coalition was just a single seat short of a majority while Klimaliste missed the threshold with receiving 0.9% of the vote.[32][33]

### Greece

In Greece, Antonis Samaras was the Minister for Foreign Affairs for the liberal conservative government of New Democracy under Prime Minister Konstantinos Mitsotakis but ended up leaving and founding the national conservative Political Spring in response to the Macedonia naming dispute, resulting in the 1993 Greek legislative election where PASOK won with its leader Andreas Papandreou making a successful political comeback, which was considered to be responsible for the Greek government debt crisis.[34][35]

### Guatemala

In 2019 the different parties to the left of National Unity of Hope (Semilla, Winaq, MLP, URNG, EG, CPO-CRD and Libre) ran with their own lists and presidential candidates. Their highest candidates Thelma Cabrera and Manuel Villacorta archived 10.3% and 5.2% respectively, combined stronger than the main conservative candidate Alejandro Giammattei 13.9% (who was elected in the run-off). If they ran together there wont have been any conservative candidate in the run-off.[36][37][38] A similar scenario happened in the 2023 election, in which four right-of-centre candidates (Manuel Conde, Armando Castillo, Edmond Mulet and Zury Ríos) gained just below 11% each, all behind Semilla's candidate Bernardo Arévalo with around 16%.

### Hong Kong

In Hong Kong, vote splitting is very common for the pro-democracy camp, which caused it to suffer greatly in many elections, including the 2016 Hong Kong legislative election and the 2015 Hong Kong local elections. Pro-democracy supporters typically have different ideologies and suffer from factional disputes that are exacerbated after the advent of localist camp. However, many have wider aggregate support fewer seats are earned than the pro-Beijing camp, an example being in Kowloon East in which pro-democracy parties got over 55% of cast ballots but won only 2 seats out of 5.[citation needed]

### Italy

Sicily is traditionally dominated by the centre-right but in the 2012 Sicilian regional election the centre-right was split between Nello Musumeci, Gianfranco Micciché, Mariano Ferro and Cateno De Luca allowing the centre-left Rosario Crocetta to win the election with just 30.5%.[citation needed]

### New Zealand

In New Zealand, there have been two notable cases of the spoiler effect. In the 1984 general election, the free-market New Zealand Party deliberately ran for office to weaken support former Prime Minister Robert Muldoon, the incumbent. The 1993 general election saw the New Zealand Labour Party's vote split by The Alliance, which has been attributed to the vagaries of the plurality vote. In response to these problems, New Zealand has since adopted mixed-member proportional representation.[citation needed]

### Nicaragua

Before the 2006 Nicaraguan presidential election, the Nicaraguan Liberal Alliance broke away from the Constitutionalist Liberal Party. This allowed Daniel Ortega to win the election with 38%, his two liberal opponents got 51% combined.[citation needed]

### Paraguay

In the 2008 Paraguayan presidential election, the candidate of the opposition alliance Fernando Lugo won the election with just 42% because Lino Oviedo ran on his own. The two right-wing candidates had 54% together but due to the split of the Colorado Party went into opposition for the first time since 1947.[citation needed]

### Philippines

In the 2004 Philippine presidential election, those who were opposed to Gloria Macapagal Arroyo's presidency had their vote split into the four candidates, thereby allowing Arroyo to win. The opposition had film actor Fernando Poe, Jr. as its candidate, but Panfilo Lacson refused to give way and ran as a candidate of a breakaway faction of the Laban ng Demokratikong Pilipino. Arroyo was later accused of vote-rigging.

### South Korea

In 1987, Roh Tae-woo won the South Korean presidential election with just under 36% of the popular vote because his two main liberal rivals split the vote. A similar scenario happened when in 1997 won by just Kim Dae-jung 40.3% because his two main conservative rivals split the vote.

### Taiwan

In the 2000 presidential election in Taiwan, James Soong left Kuomintang (KMT) party and ran as an independent against KMT's candidate Lien Chan. This caused vote-splitting among KMT voters and resulted in victory for Democratic Progressive Party's candidate, Chen Shui-bian. It is the first time in Taiwan history that the KMT did not win a presidential election, and it became the opposition party.

### United Kingdom

In the 1994 European Elections, Richard Huggett stood as a "Literal Democrat" candidate for the Devon and East Plymouth seat, with the name playing on that of the much larger Liberal Democrats. Huggett took over 10,000 votes, and the Liberal Democrats lost by 700 votes to the Conservative Party. The Registration of Political Parties Act 1998, brought in after the election, introduced a register of political parties and ended the practice of deliberately confusing party descriptions.[39]

In the run up to 2019 UK General Election, the Brexit Party, led by former UKIP leader Nigel Farage, initially put up candidates in 600 seats after a strong showing for the newly formed party in the 2019 European Elections, but days later, he reversed his position after Conservative British Prime Minister Boris Johnson stated that he would not consider an electoral pact with the Brexit Party. That was seen as benefiting the Conservative Party and disadvantaging the Labour Party.[40] Farage later encouraged voters not to vote for the Labour Party in areas that traditionally favoured it but voted to leave in the 2016 EU Membership Referendum but instead to vote tactically.[41] After the Conservatives' decisive victory, it was suggested by some media outlets and political analysts that Farage had acted as "kingmaker" and stalking horse and effectively won the election for the Tories, as Farage's decision avoided splitting the vote.[42][43]

### United States

Since 1990, the Republican Party's presidential ticket, according to the research cited below, has benefited most from the spoiler effect of the plurality voting system that chooses electors for the electoral college. The year 2000 was an especially clear case when Al Gore would likely have won without vote splitting by one or more of the third-party tickets on the ballot.[44][45] Which party benefits from a third-party ticket depends on the election and the candidates.

#### President (before 1970)

• In 1968, George Wallace ran for president as the American Independent Party's nominee and was the most recent third-party candidate to win a state.
• In 1912, Progressive Party candidate Theodore Roosevelt won almost 700,000 votes more than did the Republican incumbent, William Howard Taft.[59] Republicans, who, after Woodrow Wilson won the 1912 election, became concerned that Roosevelt might return to split the Republican vote again.[60]
• In the 1884 presidential election, the Prohibition Party's presidential nominee, former Republican Governor John St. John, took 147,482 votes, with 25,006 votes coming from New York, where Grover Cleveland defeated James G. Blaine by just 1,149 votes, allowing Cleveland to defeat Blaine in a very close contest (219-182 in the electoral college and a margin of 0.57% in the popular vote). Republicans were so angered by St. John's party switch, which caused their first presidential election defeat since 1856, that on November 27, 1884, an effigy of St. John was burned in Topeka, Kansas in front of a crowd of three thousand people.[61]

## Notes

1. ^ Examples are the first past the post electoral system and in single transferable vote or similar systems with a first-preference votes winning percentage.
2. ^ A term designed to appeal to a wider section of the public as a result of the widespread, often national support of political parties.
3. ^ More politically viable by common public sentiment which may sometimes be indicated in opinion polls.
4. ^ For example, if the Democrats had nominated both Hillary Clinton and Barack Obama for U.S. President in 2008, it would have allowed the Republican candidate (John McCain) to easily win; the voters who preferred both Clinton and Obama over McCain could not have been relied on to solve the strategy coordination problem on their own.
5. ^ For example, in the United Kingdom, UKIP have a policy of not standing parliamentary candidates where the incumbent is a committed eurosceptic member of the large Conservative Party; however, one rebel spoiler candidate from the party, Jake Baynes, led to the defeat of David Heathcoat-Amory in Wells in the 2010 United Kingdom general election by the Liberal Democrats (UK).

## References

1. ^ a b c Sen, Amartya; Maskin, Eric (2017-06-08). "A Better Way to Choose Presidents" (PDF). New York Review of Books. ISSN 0028-7504. Retrieved 2019-07-20. plurality-rule voting is seriously vulnerable to vote-splitting ... runoff voting ... as French history shows, it too is highly subject to vote-splitting. ... [Condorcet] majority rule avoids such vote-splitting debacles because it allows voters to rank the candidates and candidates are compared pairwise
2. ^ Buchler, Justin (2011-04-20). Hiring and Firing Public Officials: Rethinking the Purpose of Elections. Oxford University Press, USA. ISBN 9780199759965. a spoiler effect occurs when entry by a third-party candidate causes party A to defeat party B even though Party B would have won in a two-candidate race.
3. ^ King, Bridgett A.; Hale, Kathleen (2016-07-11). Why Don't Americans Vote? Causes and Consequences: Causes and Consequences. ABC-CLIO. ISBN 9781440841163. Those votes that are cast for minor party candidates are perceived as taking away pivotal votes from major party candidates. ... This phenomenon is known as the 'spoiler effect'.
4. ^ a b Borgers, Christoph (2010-01-01). Mathematics of Social Choice: Voting, Compensation, and Division. SIAM. ISBN 9780898716955. Candidates C and D spoiled the election for B ... With them in the running, A won, whereas without them in the running, B would have won. ... Instant runoff voting ... does not do away with the spoiler problem entirely, although it ... makes it less likely
5. ^ Heckelman, Jac C.; Miller, Nicholas R. (2015-12-18). Handbook of Social Choice and Voting. Edward Elgar Publishing. ISBN 9781783470730. A spoiler effect occurs when a single party or a candidate entering an election changes the outcome to favor a different candidate.
6. ^ Poundstone, William (2009-02-17). Gaming the Vote: Why Elections Aren't Fair (and What We Can Do About It). Farrar, Straus and Giroux. ISBN 9781429957649. IRV is excellent for preventing classic spoilers-minor candidates who tip the election from one major candidate to another. It is not so good when the 'spoiler' has a real chance of winning
7. ^ "The Spoiler Effect". The Center for Election Science. 2015-05-20. Retrieved 2017-01-29.
8. ^ "The Problem with Instant Runoff Voting". minguo.info. Retrieved 2017-01-29. After a minor party is strong enough to win, on the other hand, a vote for them could have the same spoiler effect that it could have under the current plurality system
9. ^ "RangeVoting.org - Example to demonstrate how IRV leads to 'spoilers', 2-party domination". www.rangevoting.org. Retrieved 2017-01-29. IRV means betraying your true favorite third party candidate pays off. Voting third party can mean wasting your vote under IRV, just like under plurality.
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