The two-party system has been a defining feature of American politics for over two centuries. It is a phenomenon in which two major political parties, currently the Democratic Party and the Republican Party, dominate the political landscape, making it difficult for third parties to gain significant support or achieve electoral success. This system has evolved over time, with periods of relative stability and periods of significant change.

One of the main reasons for the persistence of the two-party system is the first-past-the-post electoral system used in most US elections. In this system, the candidate who receives the most votes wins, even if they do not have a majority of the total votes cast. This creates an incentive for voters to choose one of the two major parties, as a vote for a third party candidate is often seen as a wasted vote. Additionally, the winner-takes-all system used in most state and federal legislative races reinforces the two-party system by creating a strong bias towards single-member districts where only one representative is elected.

The two-party system has also been shored up by various other factors such as campaign finance laws, ballot access restrictions, and the role of the media. For example, the Supreme Court's decision in Citizens United v. Federal Election Commission (2010) allowed for unlimited spending by outside groups, which has largely benefited the two major parties. Moreover, ballot access laws in many states make it difficult for third-party candidates to get on the ballot, further limiting their visibility and viability. Finally, the media tends to focus primarily on the horse race aspect of elections, giving disproportionate coverage to the two major parties and ignoring alternative viewpoints.