||The examples and perspective in this article deal primarily with the United States and do not represent a worldwide view of the subject. (November 2012)|
The spoiler effect is the effect of vote splitting between candidates or ballot questions[n 1] with 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. The minor candidate causing this effect is referred to as a spoiler[n 2]. However, short of any electoral fraud, this presents no grounds for a legal challenge.
Relationship with other effects 
The spoiler candidate takes votes away from a more viable[n 3] candidate or candidates, a common effect called vote splitting. Where one opposing candidate is ideologically or politically similar and therefore receives far fewer votes than other opposing candidates to the spoiler candidate, then the vote splitting has a spoiler effect.
In some cases, even though the spoiler candidate cannot win themselves, their influences upon the voters may allow the candidate to determine deliberately which of the more viable candidates wins the election — a situation known as a kingmaker scenario. In the first past the post electoral system this is particularly feasible where a spoiler candidate recommends tactical voting or runs on a false manifesto to bolster the prospects of their secretly preferred winning candidate, which in some jurisdictions and circumstances can amount to electoral fraud.
In a preferential voting system, a voter can feel more inclined to vote for a minor party or independent as their first choice and they 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 minimising 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.
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. 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.
Spoiler effect in American elections 
Presidential elections 
Bush, Gore, and Nader (2000 U.S. presidential election) 
The 2000 U.S. Presidential election is often cited as an example of the spoiler effect. In that election, Al Gore, the Democratic candidate, received more popular votes than George W. Bush, the Republican candidate, but lost in the electoral college. In the state of Florida, the final certified vote count showed Bush with just 537 more votes than Gore. Because Bush defeated Gore in Florida, he won the state, received more votes in the electoral college, and became president of the United States.
Gore supporters pointed out that, had candidate Ralph Nader, a liberal, not run in the election, the majority of the 97,421 votes he received in Florida would have been cast for Gore. Gore supporters contend that Nader's candidacy spoiled the election for Gore by taking away enough votes from Gore in Florida to swing the election to Bush. Their argument is bolstered by a poll of Nader voters, asking them for whom they would have voted had Nader not run, which said 45 percent of Nader voters would have voted for Gore, 27 percent would have voted for Bush, and the rest would not have voted.
Nader himself and many of his supporters argued that most Nader voters would either have chosen another minor party candidate or abstained from voting, had Nader not been on the ballot. It should also be noted that all other third party candidates on the ballot in Florida received more than the 537 vote difference between Bush and Gore. Still, some observers began to refer to the spoiler effect as the "Nader effect" after the 2000 election.  
Other alleged spoilers 
These are third-party candidates who have been accused of denying victory to a major nominee in U.S. Presidential Elections:
|Spoiler candidate||Election||"Denied victory" to||Winning candidate|
|James Birney||1844||Henry Clay||James Knox Polk|
|Martin van Buren||1848||Lewis Cass||Zachary Taylor|
|Peter Cooper||1876||Samuel J. Tilden||Rutherford B. Hayes|
|John St. John||1884||James G. Blaine||Grover Cleveland|
|James B. Weaver||1892||Benjamin Harrison||Grover Cleveland|
|William Howard Taft||1912||Theodore Roosevelt||Woodrow Wilson|
|George Wallace||1968||Hubert Humphrey||Richard Nixon|
|Ross Perot||1992||George H. W. Bush||Bill Clinton|
|Ross Perot||1996||Bob Dole||Bill Clinton|
|Michael Cloud||1998||John Ensign||Harry Reid|
|Ralph Nader||2000||Al Gore||George W. Bush|
Other elections 
- In 1994 moderate Republican Marshall Coleman ran for the U.S. Senate as an independent, receiving over 11 percent of the vote in an election where Democrat Chuck Robb defeated Republican nominee Oliver North by only three percent of the vote.
- As a result of the 2011 Wisconsin protests and subsequent recall elections, the Wisconsin Republican Party has encouraged spoiler candidates to run in the recall elections on the Democrat ticket in order to force the Democrats into a Primary election. Republicans argued that this will even the playing field in the recalls, as incumbents facing recall did not have the time to campaign due to their work load in the state senate.
Other countries 
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 in order to weaken support for the incumbent Prime Minister Robert Muldoon. Later on, 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 first past the post electoral system. In response to these problems, New Zealand has since adopted the mixed-member proportional voting system.
In sports, the "spoiler effect" refers to a similar phenomenon, in which a team or individual has failed to win enough games or competitions to make the playoffs or qualification rounds like the finals, but affects the playoffs or finals anyway by beating a more successful team or individual before the end of the season. For example, a baseball team that is ten games out of contention for a playoff berth could defeat a team that has a playoff berth several times. This could cause the would-be playoff team to be passed by in the rankings by the team directly behind it before the final positions at the end of the season are determined.
In individual participant sports, such as automobile racing, a racer with no hope of obtaining a championship title could prevent a racer with a chance at the title by defeating them, preventing the contending racer from earning critical points toward winning the title. Instead, the title would go to the contender directly behind him in the rankings, provided that second-tier racer is close enough to surpass and they win their own competition.
See also 
Notes and References 
- Examples are the first past the post electoral system and in single transferable vote or similar systems with a first-preference votes winning percentage
- A term designed to appeal to a wider section of the public as a result of the widespread, often national support of political parties
- More viable by common public sentiment which may sometimes be indicated in opinion polls
- 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.
- 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 United Kingdom general election, 2010 by the Liberal Democrats (UK)
- Public Disclosure Division (December 2001). "2000 Official Presidential General Election Results". Federal Election Commission. Retrieved August 30, 2010.
- Rosenbaum, David E. (February 24, 2004). "Relax, Nader Advises Alarmed Democrats, but the 2000 Math Counsels Otherwise". The New York Times (New York). Retrieved August 30, 2010.
- State Elections Offices. "2000 OFFICIAL PRESIDENTIAL GENERAL ELECTION RESULTS". Retrieved June 25, 2012.
- Bacon Jr., Karen; Tumulty (May 31, 2004). "The Nader Effect". Time. Retrieved August 30, 2010.
- Kuhn, David Paul (February 23, 2004). "The Nader Effect". CBSNews.com. CBS News. Retrieved August 30, 2010.
- Cook, Charlie (March 9, 2004). "The Next Nader Effect". The New York Times. Retrieved August 30, 2010.
- "Wisconsin GOP backs spoiler candidates in recall elections". WSJ.