9.4: The Two-Party System

The Democratic and Republican parties are not the only parties in the United States. Minor parties (most prominently the Green Party and the Libertarian Party) also field candidates in elections and have organizations and supporters. Furthermore, independent candidates unaffiliated with any party often throw their hats into the electoral ring. When it comes to winning elections, though, Democrats and Republicans dominate. In 2023, for example, all but three U.S. senators, all U.S. representatives, and all state governors were either Democrats or Republicans, as have been all presidents since 1853.

This electoral dominance of the Democratic and Republican parties is referred to as a two-party system, and it is caused by the rules of American elections. This doesn’t mean that minor-party and independent candidates are banned from participating in or winning elections in the United States. Rather, America’s two-party system evolved organically from the fact that most elected officials are chosen in single-member districts (where there can be only one winner) by plurality vote (in which the candidate with the most votes wins). Under these conditions, electoral competition tends toward the development of two major parties. This tendency is called Duverger’s law, named after the French sociologist (Maurice Duverger) who first proposed it.

To understand how Duverger’s law works, consider a typical American general election involving two major-party candidates, one Democrat and one Republican. Suppose the election is expected to be close, with each candidate winning about half of the votes cast. Now suppose a candidate from the Green Party decides to run against the Democrat and the Republican. Of the two major-party candidates, the Green prefers the Democrat (though of course he prefers himself to both of them). However, by running in the election as a third-party candidate, the Green takes away more votes from the Democrat than from the Republican (because most of his supporters would have supported the Democrat if he hadn’t run). The Green does not earn nearly enough votes to have a chance of winning, but he does earn enough to make it harder for the Democrat to win and easier for the Republican to win. By competing in the general election, the Green increases the likelihood of what he would consider the worst possible outcome: a Republican victory.

This phenomenon is known as the spoiler effect, and is a common occurrence in American elections featuring minor parties. In fact, the previous paragraph accurately describes the 2000 presidential election, in which Green candidate Ralph Nader siphoned enough votes from Democrat Al Gore to help Republican George W. Bush win the presidency. (Nader wasn’t the only reason Bush won, but the election was so close that Gore probably would have won had Nader not run.) Minor-party and independent candidates usually lack the support necessary to win, but their presence on the ballot risks “spoiling” the election by reducing the chances of victory for the major party whose views are most similar to their own.

Duverger’s law is not a law in the sense that a constitution or statute is a law. No one voted on it or decided it should be applied to the United States, nor could anyone choose to amend it like a constitution or repeal it like a statute.

Rather, Duverger’s law is more like the law of gravity: it is how party politics works naturally under the rules of American elections, whether people like it or not. In countries with different electoral rules (such as Germany or Israel) or greater regional variation (such as Canada or the United Kingdom), multiparty systems have developed just as naturally as America’s two-party system did.

The spoiler effect essentially punishes minor parties and independents for running in elections by increasing the likelihood of their least-desired electoral outcomes. In a sense, their inability to regularly win elections is a self-fulfilling prophecy: voters perceive minor parties as noncompetitive and choose not to vote for them, which makes them less competitive, which reinforces voters’ perceptions of their noncompetitiveness, and so on.