The Gibbard-Satterthwaite theorem of the 1970s proved that incentives to vote strategically exist in any non-dictatorial voting system to select between three or more alternatives. However, the extent to which these incentives exist varies widely from voting system to voting system.
The game-theoretic analysis of tactical voting usually relies on two assumptions. First, voters are short-term instrumentally rational. That is, voters are only voting in order to make an impact on one election at a time, and they understand how best to maximize the utility of the election using tactical voting. This assumption generally holds true, but there are significant cases in which it doesn't. By sacrificing their short-term interests, voters can actually maximize their long-term interests, by, demonstrating enough support for a candidate that they become viable in the next election, or by hitting vote totals that make it easier to get on the ballot or provide in the next race.
The second assumption is that voters have accurate expectations of how other voters will vote. If the information is non-existent, voters have nothing to base their decisions on but their personal preferences, and hence will vote sincerely. If the information exists, but is distorted, it may lead to voters voting tactically for inaccurate conditions.
Therefore, campaigns try hard to shape the information voters receive about the campaign. Some candidates commission their own public opinion polls. Most craft refined media strategies to shape the way voters see their candidacy. In rolling elections, where some voters have information about previous voters' preferences (e.g. presidential primaries in the United States), candidates put disproportionate resources into competing strongly in the first few stages, because those stages affect the reaction of latter stages.
In many cases, it is difficult to distinguish between tactical voting by actual voters, and it's analog in elites. For all the same reasons which the voters might decide to vote tactically, elites may decide to tactically support candidates with resources (monetary or otherwise), or by educating voters about the benifits of tactical voting. Often, a campaign can be sunk before it ever starts because it fails to convince enough sympathetic elites that the campaign is viable, and hence worth backing.
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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