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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.
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. 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.
A well-known effect of vote splitting is the spoiler effect, in which a popular candidate loses an election by a small margin because a less-popular similar candidate attracts votes away from the popular candidate, allowing a dissimilar candidate to win.
Strategic nomination takes advantage of vote splitting to defeat a popular candidate by supporting another similar candidate.
Vote splitting and electoral systems 
Different electoral systems have different levels of vulnerability to vote splitting.
Approval voting reduces the vote-splitting effect compared to plurality voting, but vote splitting still occurs because the full preferences of voters are not collected.
Runoff voting is less vulnerable to vote splitting, yet vote splitting can occur in any round of runoff voting. Although instant runoff voting (IRV) uses ranked ballots, secondary preferences are considered in the same sequence as in multiple rounds of voting, so this method does not reduce the vote-splitting effect (compared to runoff voting).
Vote splitting rarely occurs when the chosen electoral system uses ranked ballots and a pairwise-counting method, such as a Condorcet method. Vote splitting rarely occurs when using pairwise counting methods because they do not involve distributing each voter's vote among 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 that facilitates vote splitting in other voting methods.
When ranked ballots are used, a voter can vote for a minor party candidate as their first choice, and also indicate their order of preference for the remaining candidates, without regard for whether a candidate is in a major political party. For example, voters who support a very liberal candidate can select a somewhat liberal candidate as their second choice, thus minimising the chance that their vote will result in the election of a conservative candidate.
Voting methods that are vulnerable to strategic nomination, especially methods 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) where earlier rounds of voting determine the current contestants.
In the United States vote splitting commonly occurs in primary elections. The purpose of primary elections is to eliminate vote splitting among candidates in the same party. 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 only occur within each party, so vote splitting can still occur between parties in the secondary election.
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 these cases, "fringe" parties that do not meet the threshold can take away votes from larger[clarification needed] parties with similar ideologies.
Historical examples of vote splitting 
- 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.
- In 1987 Roh Tae-woo won the South Korean presidential election with just under 36% of the popular vote because his two main rivals split the vote.
- In the United States presidential elections of 1992 and 1996, spoiler candidate Ross Perot allegedly split votes away from Republican president George H.W. Bush (1992) and senator Bob Dole (1996), causing Democratic candidate Bill Clinton to win. Similarly in 2000, spoiler candidate Ralph Nader is believed to have split votes away from Democratic candidate Al Gore, contributing to the victory of Republican candidate George W. Bush.
- 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 KMT did not win in presidential election and became the opposition party.
- In the special 2003 California gubernatorial race won by Republican Arnold Schwarzenegger, which did not involve a primary election and which listed 135 candidates on the ballot, concerns about vote splitting caused the Democratic party to withdraw all but one of its major candidates, and caused the Republicans to withdraw most of their candidates. Likewise, any supporters of Republican Tom McClintock changed their mind at the last minute and voted for Schwarzenegger for fear of the Democratic candidate, Cruz Bustamante, winning.
- From 1993 to 2004, the conservative vote in Canada was split between the Progressive Conservatives and the Reform (later the Alliance) Party. This allowed the Liberal Party to win almost all the seats in Ontario during this period as well as win three successive majority governments.
- 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.
- 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 their 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.
- In the 2010 special election for the 1st congressional district of Hawaii, Republican Charles Djou won against Democratic candidates Colleen Hanabusa and Ed Case.
- 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 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%), Independent candidate Abdel Moneim Aboul Fotouh (17.5%), or Independent candidate Amr Moussa (11.1%).
- The liberal vote in Canada is currently split between the New Democratic Party and the Liberal Party. This has allowed the Conservative Party to win the last three elections in 2006, 2008 and 2011.
See also 
- Independence of clones
- Independence of irrelevant alternatives
- Spoiler effect
- Strategic nomination
- Tactical voting
- Ending The Hidden Unfairness In U.S. Elections explains why plurality and runoff voting methods are vulnerable to vote splitting.