Why Two Parties? Ambition, Policy, and the Presidency

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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.