In many systems that encourage tactical voting, notably plurality voting, voters have an incentive to avoid voting for candidates they believe have no chance of winning, regardless of whether they prefer that candidate. Instead, by expressing their preference amongst those candidates who they believe have a chance of winning, their vote has a better chance of affecting the outcome in a desirable way. Frequently, this leads to support coalescing around a few--often just two--candidates before the election occurs.
In other systems, including borda count, voters have an incentive to avoid voting for candidates they believe are close competitors to their preferred candidate, regardless of their opinion of that candidate.
Generally speaking, the more information the ballot records regarding the voters' preferences, the less useful tactical voting is. Plurality voting particularly encourages tactical voting because voters are so restricted in the information they give, whereas they are allowed to give much more information in Instant Runoff Voting. However, there are exceptions to this rule: borda count ballots record a large amount of information, but are still very conductive to tactical voting.
All forms of tactical voting require information regarding the intent of other voters. In secret ballot elections, however, information can never be perfect. Control over the flow of information, therefore, becomes of paramount importance.
In elections where most voters get their information through mass media, candidates will often have very refined media strategies intended to maximize their apperance of popularity. Some candidates will commission their own public opinion polls intended to demonstrate the wide level of support they receive. In rolling elections, such as the primary election for the President of the United States, candidates will put disproportionate amounts of resources into competing strongly in the first few stages of the election, in order to appear viable in later stages.
In plurality vote and similar systems, primary elections within a political party are often used to simplify tactical voting. Members of a party, joined by roughly similar political views, hold an election amongst themselves to determine which candidate to support. This process mirrors the organic process of weeding out the candidates with the smallest amount of support, but because it is formal, the results tend to be more binding.
When a party does not have enough support within themselves to elect a candidate in the general election, members of the party will sometimes vote tactically for a candidate they believe has a greater chance of drawing support from outside the party, and hence, a greater chance to get elected.
Tactical voting is quite well known in United Kingdom elections. In England, there are three parties that are represented in the Parliament: the Labour party, the Conservative? party and the [Liberal Democrats]?. Of these three, Labour and the Liberal Democrats are most similar. 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.
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