Explanation:
As noted, Republicans and Democrats have dominated electoral politics since the 1860s. This unrivaled record of the same two parties continuously controlling a nation’s electoral politics reflects structural aspects of the American political system as well as special features of the parties.
The standard arrangement for electing national and state legislators in the United States is the “single-member” district system, wherein the candidate who receives a plurality of the vote (that is, the greatest number of votes in the given voting district) wins the election. Although a few states require a majority of votes for election, most officeholders can be elected with a simple plurality.
Unlike proportional systems popular in many democracies, the single-member-district arrangement permits only one party to win in any given district. The single-member system thus creates incentives to form broadly based national parties with sufficient management skills, financial resources and popular appeal to win legislative district pluralities all over the country. Under this system, minor and third-party candidates are disadvantaged. Parties with minimal financial resources and popular backing tend not to win any representation at all. Thus, it is hard for new parties to achieve a viable degree of proportional representation, and achieve national clout, due to the “winner-take-all” structure of the U.S. electoral system.
Why two instead of, say, three well-financed national parties? In part because two parties are seen to offer the voters sufficient choice, in part because Americans historically have disliked political extremes, and in part because both parties are open to new ideas (see below).
