Vote splitting

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. 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. The spoiler effect is the effect of vote splitting between candidates or ballot questions 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. The minor candidate causing this effect is referred to as a spoiler. The problem also exists in two-round system and instant-runoff voting 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 ).