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.
Cardinal voting methods are immune to vote splitting, since each candidate is rated independently of each other. Pairwise-counting Condorcet methods minimize vote splitting effects. Plurality-runoff voting methods (like Exhaustive ballot, Two-round system/Top-two primary,Instant-runoff voting,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.
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. As a result, the notion of vote splitting is controversial because it can discourage third party candidates.
Strategic nomination takes advantage of vote splitting to defeat a popular candidate by supporting another similar candidate.
Different electoral systems have different levels of vulnerability to vote splitting.
Vote splitting most easily occurs in plurality voting because the ballots only gather the "least worst" preference of the voter. 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 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 only occur within each party, 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 these cases, "fringe" parties that do not meet the threshold can take away votes from larger[clarification needed] parties with similar ideologies.
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.
Runoff voting is less vulnerable to vote splitting compared to plurality voting, 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 this method does not reduce the vote-splitting effect.
Vote splitting rarely occurs when the chosen electoral system uses ranked ballots and a pairwise-counting method, such as a Condorcet method. Pairwise counting methods 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.
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.
Cardinal voting methods require an independent score to be given to candidates, as opposed to a ranking. The three primary Cardinal Voting methods are Approval Voting, with a range between 0-1, Score Voting where there's an arbitrary range, and STAR voting.
All cardinal voting methods are immune to vote-splitting, as each candidate is evaluated independently of each other candidate.
While the Liberal and National Parties tend to avoid three-cornered contests they do occur when there is a dispute of which party has claim to seats in question. Australia has the system of preferential voting and in three-cornered contests the Liberal and National Parties would preference each other while the Australian Labor Party would normally preference the Liberal Party ahead of the National Party as they deem the Liberal Party to be less conservative than the National Party.
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
IRV is subject to something called the "center squeeze." A popular moderate can receive relatively few first-place votes through no fault of her own but because of vote splitting from candidates to the right and left. ... Approval voting thus appears to solve the problem of vote splitting simply and elegantly. ... Range voting solves the problems of spoilers and vote splitting
You likely have opinions about all those candidates. And yet, you only get a say about one.