The U.S. employs a fairly complicated system of electing a President. It involves the process of nominating a candidate and then a two-step procedure of electing the Chief Executive via the Electoral College. Combined with other factors, this process effectively ensures the enduring stability of the two-party system at the expense of non-major parties.
Elections begin as political parties nominate several candidates each. The parties hold primaries in each state, allowing voters to express their support for the nominees. Then the national convention with delegates from states and territories, as well as other representatives, such as top party leaders, choose the party’s nominee for the election. On Election Day, people vote, obliging the state’s electors to support the nominee with the most votes. After that, the Electoral College chooses the President by the majority of votes.
The United States uses a winner-take-all approach in presidential elections, meaning that the candidate who has the most votes in the state receives all votes of its electors. It differs from the proportional system, where each candidate would receive several votes proportional to his or her electoral support. Third parties, being smaller and less influential, have no hope to win the majority in a state and are effectively excluded. The single-member district system tends to produce such results in general, as winning a majority in a district is virtually impossible when competing with major parties.
Other rules, both institutional and not, make third-party chances even slimmer. The two-steps process ensures that, even if a party enjoys substantial popular support, it will not count in the Electoral College unless it wins in some states. Some states also oblige third-party candidates to amass a certain number of signatures for a petition to run, unlike the major party candidates. Non-institutional factors are also important: since people know that a third party has no chances of winning an election in the winner-take-all system, they will eventually stop supporting it to not waste votes. Additionally, the relative prosperity of the USA makes it harder to create small but stable parties that rely on disgruntled minorities.
To summarize, the U.S. presidential election process supports the two-party system. The winner-take-all approach, single-member districts, two-step voting process, and limitations imposed upon third-party nominees contribute to that. People’s unwillingness to waste votes on unsuccessful parties and the absence of strongly dissatisfied and politically coherent minorities also ensure the stability of the two-party system.