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The spoiler effect is a term to describe the effect a candidate can have on a close election, in which their candidacy results in the election being won by a candidate dissimilar to them, rather than a candidate similar to them.

It can also refer to a similar phenomenon in sports: when a team has failed to win enough games to make the playoffs, it can often affect the playoffs anyway, by beating a more successful team before the end of the season.

One often cited example of the spoiler effect at work was the 2000 U.S. Presidential election. In that election, George W. Bush and Al Gore had a very close election in many states, with neither candidate winning a majority of the votes. In Florida, the final certified vote totals show Bush won just 537 more votes than Gore, thus winning the state and the presidency. Many Gore supporters believed that most of the 97,421 votes that went to Ralph Nader in that state would have likely been votes for Gore had Nader not been in the election. They contend that Nader's candidacy "spoiled" the election for Gore by taking away enough votes from Gore in Florida and many other states (in particular, New Hampshire being the allegation most statistically supportable) only to allow Bush to win. Nader supporters say he had to run to protest Gore's positions, and the punishment of defeat is a powerful weapon that democracy allows and encourages. Without Nader on the ballot, angry anti-Gore voters might as well have voted for Bush to punish Gore. However, Nader himself and many of his supporters argue that most Nader voters would have chosen another minor party candidate, or refrained from voting altogether, had he not been on the ballot. Some observers began to refer to the spoiler effect as the Nader effect after the 2000 election.

A similar effect was observed in 1987, when Roh Tae-woo won the South Korean presidential election with just under 36% of the popular vote after his two main rivals split the vote.

The spoiler effect is one of the components contributing to the effect known as Duverger's law, which states that the first-past-the-post election system creates and preserves a two-party system.

Preferential voting methods can lessen this effect. A voter in a preferential voting system can vote for a minor party candidate first and still record a preference between the remaining (major party) candidates. (In the Nader example above, Nader voters could have selected Nader as their first choice, and then ranked Bush and Gore second and third as they saw fit.)

Approval voting can also reduce this effect.

Vote splitting is closely related to the spoiler effect. Vote splitting is the distribution of votes among similar candidates in a situation that decreases the likelihood of winning for any of the similar candidates.

As an example of vote splitting, if 30 percent of voters prefer candidate A, another 30 percent prefer a similar candidate B, and the remaining 40 percent prefer a dissimilar candidate C, then plurality voting identifies candidate C as the winner, even though a majority of voters (60 percent) prefer either candidate A or candidate B.

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.^[1]

Plurality voting is especially vulnerable to vote splitting. Runoff voting methods are less vulnerable, and pairwise-counting Condorcet methods minimize vote splitting effects.^[1]

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 and Turkey. In these cases, "fringe" parties that do not meet the threshold can take away votes from larger parties with similar ideologies.

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

Arrow's impossibility theorem shows 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. The local independence of irrelevant alternatives criterion is similar to IIA, but which can be passed by some ranked ballot methods.

These are third-party candidates who feasibly could have denied victory to a major nominee.

• Charles Cotesworth Pinckney, 1800
• James Birney, 1844
• Martin van Buren, 1848
• Theodore Roosevelt, 1912
• Henry Ross Perot, 1992 and 1996
• Ralph Nader, 2000

The spoiler effect also sometimes occurs in congressional elections and elections for state offices.

• The Decoy effect is related, but suggests (for example) that, in 2000, the presence of Nader pulled votes from Bush to Gore, by making Gore appear more moderate by comparison.