In voting systems, tactical voting (or strategic voting or sophisticated voting) occurs when a voter supports a candidate other than his or her sincere preference in order to prevent an undesirable outcome.
It has been shown by the Gibbard-Satterthwaite theorem that any voting method which is completely strategy-free must be either dictatorial or nondeterministic (that is, might not select the same outcome every time it is applied to the same set of voter preferences). For instance, the random ballot voting method, which randomly selects the ballot of a single voter and uses this to determine the outcome, is strategy-free, but may result in different choices being selected if applied multiple times to the same set of ballots.
However, the type of tactical voting and the extent to which it affects the timbre of the campaign and the results of the election vary dramatically from one voting system to another.
It is also a vital strategy within cumulative voting.
In the 1997 United Kingdom general election in Winchester, Mark Oaten (Liberal Democrat) beat the incumbent Conservative MP Gerry Malone with a majority of 2 votes. Malone successfully challenged the election in the High Court, which declared it void. A by-election was held which returned Mark Oaten as MP, this time with a larger majority of 21,556. This was due to the majority of Labour voters voting Liberal in the by-election.
In United Kingdom elections, there are three main parties represented in the Parliament: the Labour party, the Conservative party and the Liberal Democrats. Of these three, Labour and the Liberal Democrats are considered most similar by many voters. Many people who prefer the Liberal Democrats vote for the Labour candidate where Labour is stronger and vice-versa where the Liberal Democrats are stronger, in order to prevent the Conservative candidate from winning.
In the 1997 UK general election, Democratic Left helped Bruce Kent set up GROT - Get Rid Of Them - a tactical voter campaign whose sole aim was to help prevent the Conservative Party from gaining a 5th term in office. This coalition was drawn from individuals in all the main opposition parties and many who were not aligned with any party. While it would be hard to prove that GROT swung the election itself, it did attract significant media attention and brought tactical voting into the mainstream for the first time in UK politics. In 2001, the Democratic Left's successor organisation the New Politics Network organised a similar campaign tacticalvoter.net Since then tactical voting has become a real consideration in British politics as is reflected in by-elections and by the growth in sites such as tacticalvoting.com who encourage tactical voting as a way of defusing the two party system and empowering the individual voter. In the 2005 UK General Election individuals set up tacticalvoting.net to balance the tactical voting debate.
In the Ontario general election, 1999, strategic voting was widely encouraged by opponents of the Progressive Conservative government of Mike Harris. This failed to unseat Harris, and succeeded only in suppressing the New Democratic Party vote to a historic low.
In the Canadian general election, 2004 and to a lesser extent in the Canadian general election, 2006, strategic voting was a concern for the federal New Democratic Party. In the 2004 election, the governing Liberal Party was able to convince many New Democratic voters to vote Liberal in order to avoid a Conservative government. In the 2006 elections, the Liberal Party attempted the same strategy, with Prime Minister Paul Martin asking New Democrats and Greens to vote for the Liberal Party in order to prevent a Conservative win. The New Democratic Party leader Jack Layton would respond by asking voters to "lend" their votes to his party, suggesting that the Liberal Party would be bound to lose the election regardless of strategic voting.
In the 2006 local elections in London, tactical voting is being promoted by sites such as London Strategic Voter in a response to national and international issues. The question of whether this approach acts to undermine local democracy is receiving much debate.
In Northern Ireland, it is widely thought that (predominantly Protestant) Unionist voters in Nationalist strongholds have voted for the Social Democratic and Labour Party (SDLP) to prevent Sinn Féin from capturing such seats.
Academic analysis of tactical voting is based on the rational voter model, derived from rational choice theory. In this model, voters are short-term instrumentally rational. That is, voters are only voting in order to make an impact on one election at a time (not, say, to build the political party for next election); voters have a set of sincere preferences, or utility rankings, by which to rate candidates; voters have some knowledge of each other's preferences; and voters understand how best to use tactical voting to their advantage. The extent to which this model resembles real-life elections is the subject of considerable academic debate.
An example of a rational voter strategy is described by Myerson and Weber . The strategy is broadly applicable to a number of single-winner voting systems that are additive point systems, such as Plurality, Borda, Approval, and Range. The strategy is optimal in the sense that the strategy will maximize the voter's expected utility when the number of voters is sufficiently large.
This rational voter model assumes that the voter's utility of the election result is dependent only on which candidate wins and not on any other aspect of the election, for example showing support for a losing candidate in the vote tallies. The model also assumes the voter chooses how to vote individually and not in collaboration with other voters.
Given a set of k candidates and a voter let:
Then the voter's prospective rating for a candidate i is defined as:
The gain in expected utility for a given vote is given by:
The gain in expected utility can be maximized by choosing a vote with suitable values of vi, depending on the voting system and the voter's prospective ratings for each candidate. For specific voting systems, the gain can be maximized using the following rules:
An important special case occurs when the voter has no information about how other voters will vote. This is sometimes referred to as the zero information strategy. In this special case, the pij pivot probabilities are all equal and the rules for the specific voting systems become:
Myerson and Weber also describe voting equilibria that require all voters use the optimal strategy and all voters share a common set of pij pivot probabilities. Because of these additional requirements, such equilibria may in practice be less widely applicable than the strategies.
Because tactical voting relies heavily on voters' perception of how other voters intend to vote, campaigns in electoral systems that promote compromise frequently focus on affecting voter's perception of campaign viability. Most campaigns craft refined media strategies to shape the way voters see their candidacy. During this phase, there can be an analogous effect where campaign donors and activists may decide whether or not to support candidates tactically with their money and labor.
In rolling elections, or runoff votes, where some voters have information about previous voters' preferences (e.g. presidential primaries in the United States, French presidential elections), candidates put disproportionate resources into competing strongly in the first few stages, because those stages affect the reaction of latter stages.
Some people view tactical voting as providing misleading information. In this view, a ballot paper is asking the question "which of these candidates is the best?". This means that if one votes for a candidate who one does not believe is the best, then one is lying. British Labour Party politician Anne Begg considers tactical voting dangerous:
Tactical voting is commonly regarded as a problem, since it makes the actual ballot into a nontrivial game, where voters react and counter-react to what they expect other voters' strategies to be. British mathematician Charles Dodgson (a.k.a. Lewis Carroll) was, apparently, the first to acknowledge this fact. A game such as this might even result in a worse alternative being chosen, because most of the voters used it as a strategic tool.
Though Arrow's impossibility theorem and Gibbard-Satterthwaite theorem prove that any useful voting system is prone to some kind of manipulation, some use game theory to search for some kind of "minimally manipulatable" voting schemes.
Game theory can also be used to analyze the pros and cons of different methods. For instance, under purely honest voting, Condorcet method-like systems tend to settle on compromise candidates, while Instant-Runoff Voting favors those candidates which have strong core support - who may often be more extremist. An electorate using one of these two systems but which (in the general or the specific case) preferred the characteristics of the other system could consciously use strategy to achieve a result more characteristic of the other system. Under Condorcet, they may be able to win by "burying" the compromise candidate (although this risks throwing the election to the opposing extreme); while under IRV, they could always "compromise". It could be argued that in this case the option to vote tactically or not actually helps the electorate express its will, not only on which candidate is better, but on whether compromise is desirable. (This never applies to "sneakier" tactics such as push-over.)
Tactical voting greatly complicates the comparative analysis of voting systems. If tactical voting were to become significant, the perceived "advantages" of a given voting system (that is, tending towards compromise or favoring core support) could turn into disadvantages - and, more surprisingly, vice versa.
Due to the especially deep impact of tactical voting in first past the post electoral systems, some argue that systems with three or more strong or persistent parties become in effect forms of disapproval voting, where the expression of disapproval in order to keep an opponent out of office overwhelms the expression of approval to elect a desirable candidate. Ralph Nader refers to this as the "least worst" choice, and argues that the similarity of parties and the candidates in first past the post systems grows stronger due to the need to avoid this disapproval.
One often-overlooked flaw in the first past the post system is that, in single member districts, voters can invariably select only one candidate, whilst in multi-member districts they can never select more candidates than the number of seats. Approval voting, by contrast, allows voters to cast a vote for as many candidates as they wish. This in turn allows for "voting against" a certain despised candidate without having to guess at who the strongest contender is to defeat that candidate. Such a system would also serve to reduce the spoiler effect.
More sophisticated tactics may be practicable where the number of candidates, voters and/or seats to be filled is relatively small.
Some forms of STV allow tactical voters to gain an advantage by listing a candidate who is very likely to lose in first place, as a form of pushover. Meek's method essentially eliminates this strategy.
The term "tactical unwind" is used by some political scientists and commentators to refer to the phenomenon when tactical voting takes place in one general election but in subsequent elections voters revert to their normal patterns.