During times of great political turmoil or Washington gridlock, Americans often toy with the idea of putting an end to the two party system by creating a viable third party. However, despite the periodic creation of a variety of small alternative parties, America effectively retains its two party system. This is not a actually result of law or tradition; it is an almost inevitable effect of the American voting system.
America uses a style of voting known as the first past the post system. In this system, the winner of a plurality of the votes gets full representation for that district. In other words, if a Democratic candidate for President got 65 percent of the votes in a state, and a Republican candidate got 35 percent of the votes in it, then all of the votes of that state would go to the Democratic candidate, not just the 65 percent who voted for him or her. The proportional representation system, on the other hand, awards votes for a district in proportion to the number of votes each candidate got. The effect of the first past the post system of voting is to create a two party system. This is known as Duverger's Law.
This is because the first past the post system encourages voters to vote strategically. Voters will always vote for one of the two candidates who are most likely to win, because a vote for any candidate with fewer votes than that is probably going to be wasted. This keeps the great majority of the votes restricted to the two most popular and powerful political parties. It naturally discourages the development of a multi-party political system.
There is also the fact that in this kind of voting system, voting for third party candidate stands a good chance of increasing the odds of an undesirable election outcome. If a voter is strongly opposed to one of the two major candidates, but prefers a candidate from third party, then voting for that candidate would probably increase the chances that the candidate he or she opposes gets elected. This happens when the candidate from the third party is enough like one of the top two to draw votes away from the candidate from that party. This is what happened during the presidential election in 2000, when the popular consumer rights advocate Ralph Nader ran as a candidate for the Green Party. The Green Party had many beliefs and goals in common with the Democrats, so his candidacy cost Al Gore, the Democratic candidate, enough votes to let George Bush, the Republican candidate, win.
Third parties do sometimes still play a significant role in American politics, though. They can act as spoilers, as Nader did. They can also bring new ideas into the political arena, or show the two major parties that there is a lot of support for a particular issue, just as the Prohibition Party did in the early 20th century. This can change the course of American politics. The first past the post system means that America will always be a two party system, for the most part, but other parties do still serve a purpose.