Why Two Parties? Ambition, Policy, and the Presidency - John Aldrich publishes with Daniel Lee
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.