In most of the United States, the general election (at every level) is mostly a two-party race. In 2014, there were thirty-four races in which the winning candidate got less than 50%. In only two of these races did the winning candidate get beneath 45%. In only 11 of these races did the loser get below 45%. In ten of these races, it is probable that the minor part candidates may have altered the winner of the race. Given the rareness of such races, strategic voting is normally not viewed as a significant issue in the general election in the U.S., but it is a significant issue in the primary and in elections in other countries.
Starting with other countries, the two countries with the most similar election system to the U.S. are the United Kingdom and Canada. Both use a first-past-the-post system for parliamentary elections, just like most states use for Congressional and Senate elections. The difference is that — unlike the U.S. — Canada and the U.K. have, at least, three major parties and some parties with regional strength.
In the last U.K. election, the Conservatives won 330 seats out of 650 seats to get a majority. Out of the 650 seats, the winning candidate got less than 45% in 68 seats, and failed to get a majority in 97 seats. The Conservatives won 40 of those seats.
In the upcoming Canadian election, the current estimates by the folks at threehundredeight.com have one party getting a majority in only 93 of the 338 seats. In another 71 seats, one candidate is close to 50% with the other candidates significantly behind. Combining those two categories together, the Conservatives have a narrow plurality of 73 seats to 70 seats for the Liberals, 20 for the New Democrats, and 1 for the Green. In another 50 seats, there is a clear leading candidate (margin of more than 10%), but no clear runner-up (less than 10% between second and third). Adding this category to the other two categories, the Conservatives lead with 91 seats to 80 for the Liberals, 42 for the New Democrats, and 1 for the Greens. With two weeks left to the election, there are 88 seats that look like two-way races between two of the three major parties and 10 races that look like a two-way race between one of the three major parties and a minor party/independent. Additionally, there are 10 races that look like a close three-way race (8 involving the three major party, and 2 involving the Bloc Quebecois and two major parties), 2 four-way races and even 1 five-way race. Particularly in the two-way races, supporters of the other major party have to make the choice of whether to vote strategically (the 88 races break down as 38 Liberal-Conservative races, 20 Liberal-New Democrats races and 30 Conservative-New Democrat races) and what vote makes the most sense strategically. (Normally, the Conservatives are close politically to the Liberals and the Liberals are slightly closer to the New Democrats, but with the race this close, any of the three parties could end up first nationally. Based on current splits in these races, the Conservatives would end up at 124, Liberals would end up at 116, and the NDP would end up at 87. However, if Liberals vote for New Democrats over Conservatives, New Democrats vote for Liberals over Conservatives, and Conservatives vote for Liberals over New Democrats, Liberals would end up at 142, Conservatives would end up at 94, and New Democrats would end up 79. With the race as close as it is nationally, the winner in two weeks may be determined by strategic voting in the 100 closest races.
While the U.S. does not have this issue in the general election, it does arise in primaries — particularly when you have as many candidates running as the Republicans do in the presidential race. If you are a Santorum or a Huckabee supporter heading into the early primaries, do you keep on supporting your first pick candidate (hoping that in doing so they will get just enough votes to stay alive into the March primaries) or do you switch to your second choice candidate in the hopes that at least one candidate who is philosophically close to you will gain enough momentum to make to March) or do you switch to a candidate that is even more distant from you to deny a different candidate a clear win. In the primaries, the difference could very well come down to what supporters of the trailing candidates do — especially among voters who do not understand the delegate selection rules (and may switch from a candidate running in third or fourth but doing well enough to get delegates to make a difference in who wins between the top two candidates).