Spoiler effect

The spoiler effect is a term to describe the effect of a third party candidacy on a close election, possibly tipping the balance between two relatively close leading candidates. The effect is unique to the first-past-the-post election system, in which the candidate that receives a plurality of votes (and not necessarily a majority of votes) wins the election.

One vivid 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. Many Gore supporters contended that the votes that went to Ralph Nader, a popular third-party candidate, 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 many states to give Bush enough votes to win the electors in those states.

The spoiler effect is a consequence of Duverger's Law, which states that the first-past-the-post election system reinforces the two-party system. Much research has been done on this subject, and more can be found in the Duverger's law Wikipedia entry.