Why is US politics dominated by two parties?

The dominance of Democrats and Republicans results from electoral rules that disadvantage third parties, the strategic behaviour of voters and donors, and the broad-coalition nature of the two major parties. This pattern is explained by Duverger's Law, which describes how plurality voting systems tend toward two-party competition.

Duverger's Law

Political scientist Maurice Duverger observed that single-member plurality electoral systems (like most US elections) tend to produce two-party systems. This occurs through two mechanisms:

Mechanical effect: Third parties that win votes but not pluralities receive no seats or electoral votes. Even significant vote shares translate to zero representation under winner-take-all rules.

Psychological effect: Voters, donors, and media anticipate this outcome and strategically concentrate support on viable candidates, further marginalising third parties.

The Ross Perot example

In 1992, independent candidate Ross Perot won 18.9% of the popular vote—the strongest third-party performance in decades. However, he received zero electoral votes because his support was distributed across states rather than concentrated enough to win any state outright. This illustrates how the Electoral College's state-by-state winner-take-all system punishes geographically dispersed support.

Institutional reinforcement

Beyond electoral rules, the two-party system is reinforced by:

• Ballot access laws: State-by-state requirements for petition signatures and deadlines favour established parties
• Debate inclusion: Commission rules historically required 15% polling support to participate
• Campaign finance: Donors concentrate resources on viable candidates
• Primary elections: Allow ideological competition within parties, reducing demand for new parties

Third parties still matter

Though rarely winning, third parties influence American politics by introducing issues (Populists, Progressives), affecting outcomes (Perot 1992, Nader 2000), and signalling voter dissatisfaction. Some analysts argue they function as "ideological laboratories" whose successful ideas get absorbed by major parties.