The United States has only two major political parties: the Democrats and the Republicans. These parties have a duopoly, meaning that they share almost all the political power in the country.
In the United States, a candidate wins the election by gaining a plurality, or more votes than any other candidate. This is a winner-take-all system because there is no reward for the party or candidate that finishes second. Parties aim to be as large as possible, smoothing over differences among candidates and voters. There is no incentive to form a party that consistently gets votes but cannot win an election. As a result, two political parties usually dominate plurality electoral systems to the disadvantage of smaller third parties, just as the Democrats and the Republicans dominate the American political system. No one person or organization prevents third parties from forming, but the plurality system itself usually hinders their efforts to win votes.
The United States also has mostly single-member districts, meaning that each legislative district sends only one member to the legislature. There is no benefit to finishing second. Some countries use multiple-member districts, which makes it easier for minor parties to succeed because there are more members winning seats in the legislature.
The Electoral College exacerbates the winner-take-all system because in all but two states, whoever wins the most popular votes wins all of the state’s electoral votes in the presidential election. The electoral rules favor a two-party system, and minor parties have a very difficult time competing in such a system. Even successful third-party candidates often fail to get a single electoral vote.