Spoiler candidate

The spoiler effect is the effect of vote splitting between candidates or ballot questions 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. However, short of any electoral fraud, this presents no grounds for a legal challenge.

The spoiler effect is a problem in plurality voting systems because they enable a candidate to win with less than half of the vote.

The problem also exists in Instant-runoff voting, though it is reduced, because voters are allowed to rank their candidate choices, with their vote transferring to their second choice if their first choice does not win, and to their third choice if their second choice does not win, and so on. Other preferential voting or ranked ballot voting systems also suffer from variations of the spoiler effect, as they fail the IIA criterion (see §Mathematical definitions).

The problem does not exist in cardinal voting methods like Approval voting, Score voting, or Majority judgment, since the rating of each candidate is independent of the ratings of other candidates.

The spoiler candidate takes votes away from a more viable 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 enable the candidate to determine deliberately which of the more viable candidates wins the election — a situation known as a kingmaker scenario. With a first-past-the-post voting 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.

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