• American politics has always been dominated by two parties
• Structural factors in the political system (especially single-member district voting) favor a two-party (rather than multiparty) system

Well, in reality, there are a lot of political parties. Many of us get so absorbed in the contest between Democrats and Republicans that we forget that several other parties regularly run candidates for office. In the 2008 presidential election, for example, the nominees of as many as ten parties were listed on some state ballots. But it is also true that most of these parties rarely win elections or have much of an impact on political races. For most of our history, we have had, for all intents and purposes, a two-party system—a political process dominated by just two political parties.

Scholars attribute this fact to several reasons. Some emphasize certain structural features of our political system. Most of our elections, for example, occur in "single-member districts." This means that we elect one person, not several people, to fill a position. This gives the large established parties an advantage, because only parties that can win 50% of the votes in an election will ever win a seat; a third party might win 20% of the votes nationwide, but win zero seats in Congress because it never hit a majority in any particular district. If seats in Congress were allotted using a different system—giving each party a proportion of seats equal to the proportion of votes won nationwide—a minor party would have a much better chance of gaining representation.

In addition, over the past century, Democrats and Republicans have joined forces to diminish the threat of minor parties to their domination of the political process. In many states, they have passed electoral laws that complicate the ability of minor parties to even get their candidates' names on the ballot.

But while these structural features of our system hinder minor parties, the greatest reason for the persistence of a two-party system is the ideological consensus in the United States. Americans may disagree on policy details, but the vast majority agree on fundamental principles. Very few people in America question the basic premises of our free-market economy or the importance of maintaining a strong national defense. Across the political spectrum, people agree that the federal government has a responsibility to address economic and natural crises and to provide some degree of assistance and security to the poor, aged, and infirmed. The controversial ideas of one era—such as Social Security or racial integration—have tended to become part of the consensus in the next era. And consequently, the debates that persist are usually over the details—for example, whether Social Security funds should be partially privatized or racial integration is best advanced through affirmative action.