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Why Two Parties? Ambition, Policy, and the Presidency*

  • John H. Aldrich and Daniel J. Lee

Duverger’s Law suggests that two parties will dominate under first-past-the-post (FPTP) within an electoral district, but the law does not necessarily establish two-party competition at the national level. United States is unique among FPTP countries in having the only durable and nearly pure, two-party system. Following this observation, we answer two questions. First, what contributes to the same two parties competing in districts all across the country and at different levels of office? Second, why is the US two-party system so durable over time, dominated by the same two parties? That is, “Why two parties?” As an answer, we propose the APP: ambition, the presidency, and policy. The presidency with its national electorate and electoral rules that favor two-party competition establishes two national major parties, which frames the opportunity structure that influences party affiliation decisions of ambitious politicians running for lower offices. Control over the policy agenda helps reinforce the continuation of a particular two-party system in equilibrium by blocking third parties through divergence on the main issue dimension and the suppression of latent issue dimensions that could benefit new parties. The confluence of the three factors explains why the United States is so uniquely a durable two-party system.

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John H. Aldrich, Professor, 281 Gross Hall, 140 Science Drive, Box 90204, Durham, NC 27708-0204 ( Daniel J. Lee, Assistant Professor, Department of Political Science, University of Nevada, WRI B227, Box 455029, 4505 S Maryland Pkwy, Las Vegas, NV 89154-5029 ( The authors thank anonymous reviewers for their insightful comments and suggestions. The authors are responsible for all remaining errors. We also thank participants at a conference held by the Center for the Advanced Study of India at the University of Pennsylvania, Comparing Elections and Electoral Systems in North America and India (2011).

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Political Science Research and Methods
  • ISSN: 2049-8470
  • EISSN: 2049-8489
  • URL: /core/journals/political-science-research-and-methods
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