One of the ways in which plurality voting (sometimes called first-past-the-post) can fail to capture the preferences of voters is through “vote splitting.” Vote splitting means that two candidates who are liked equally by a majority of voters, or where voters like both better than the other candidates are less likely to be elected than a candidate who is disliked by the majority.
Vote splitting is possible for any election or referendum that has more than two options and where the one that has most votes wins. For example, in 1969 when the Ontario towns of Fort William and Port Arthur chose to amalgamate, residents were asked to vote for one of three new names: “Lakehead,” “The Lakehead,” or “Thunder Bay.” The name “Thunder Bay” won by a 568 vote margin with 15,870 votes, “Lakehead” got 15,302, and “The Lakehead” got 8,377. The winning name had 40% of the votes because the other two candidates were so similar.
Suppose you are one of three candidates in an election. Polls say that 36% of voters intend to vote for you, while 38% intend to vote for candidate A and 26% intend to vote for candidate B. Rather than convincing other voters to support you, you focus on convincing some supporters of candidate A to vote for candidate B who has a similar platform. If just 3% of voters decide to vote for candidate B rather than candidate A, you will win by 36% to 35% to 29%, without needing to convince any more people to vote for you. Vote splitting is a simple low-cost electoral strategy, but it means the result may not be the preferred choice of most voters.
The strategy is legal, but in 1995, an illegal version of the scheme was used in Manitoba, where a political party financed independent candidates to split the vote of candidates of another party.
Many electoral systems are susceptible to some form of vote splitting, including plurality voting like the one used in Canada, Mixed Member Proportional (MMP), and any proportional system with a minimum threshold.
Run-off voting with two or more rounds or voting, and ranked ballots (sometimes called instant run-off) solve the problem of vote splitting. With ranked ballots, you mark your first, second, and third choice. If your first choice comes in last, and no other candidate is the first choice of more than half the voters, then your second choice ballot is used, ensuring that the winner has the majority either of first-choice ballots, or failing that of first and second choice ballots and so on.
When ranked ballots are used, it is not possible to win with vote-splitting, whether the vote splitting occurred naturally or was a deliberate strategy, or an illegal scheme. The only way to win is to be chosen by an absolute majority of at least 50% of voters.