In winner-take-all (or plurality-take-all), the candidate with the largest number of votes wins, even if the margin of victory is extremely narrow or the proportion of votes received is not a majority. Unlike in proportional representation, runners-up do not gain any representation in a first-past-the-post system. In the United States, systems of proportional representation are uncommon, especially above the local level, and are entirely absent at the national level.

American legislators have traditionally had wide discretion to vote as they or their constituents please. A Democrat representing a rural area can be pro-life and anti-gun control; a Republican representing a suburban district can be pro-choice and pro-environment. Thus, even though there are only two parties represented in most American legislatures, there are different shades of opinion.

In America, if an interest group is at odds with its traditional party, it has the option of running sympathetic candidates in primaries. If the candidate fails in the primary and believes he has a chance to win in the general election he may form or join a third party.