The Gibbard-Satterthwaite? theorem of the 1970s proved that any minimally useful voting system has some form of tactical voting, but the extent to which it effects the timbre and results of the campaign varies dramatically from system to 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 use tactical voting to their advantage. This assumption is generally true in real life, but there are significant cases in which it doesn't. Sometimes voters will vote for a candidate they know will lose in the current election in order to show that they're viable in the next election.
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 voters, and it's analog before the election. For all the same reasons that voters might decide to vote tactically, campaign donors and activists may decide to tactically support or not support candidates with their money and labor, thus leading to results similar to those caused by tactical choices by the voters. A campaign can be sunk before it ever starts because it fails to convince enough sympathetic people that the campaign is viable, and hence worth backing.
Tactical voting is quite well known in United Kingdom elections. There are three main 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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