Why does the United States have a two-party political system?

The United States has only two major parties primarily because of the first-past-the-post electoral system. The U.S. single-member district/plurality electoral system requires a party to get a majority of the votes in a district in order to win, and the winner takes it all. Coming in second (even if only by one vote) does not count, as it would in a proportional representation system. Neither the Constitution nor state laws prohibit minor parties, nor do they mandate only two major parties. One of the criticisms of the American party system is that having only two major parties oversimplifies the electoral system, presenting voters with only two choices instead of a wide range of options. The United States has two major parties, the Democrats and the Republicans. Minor parties do exist, such as the Green Party, which is concerned with the environment, and the Reform Party, which is concerned with reforming government and paying down the national debt. However, only one "third" party in American history (the Republican Party) became strong enough to replace one of the two major parties (the Whig Party).