Explaining variation in the competitiveness of U.S. Senate elections, 1922-2004

We develop and test predictions about the factors determining the competitiveness of elections to the U.S. Senate. To do so, we deliberately abstract away from candidate-specific conditions that have often been used to study political competitiveness in order to focus on basic structural features of the electoral landscape. In our framework, party-specific constraints on the ideological positioning of local candidates, linked to the national party organization and its contributors, interact with the heterogeneity of state electorates to determine the number of highly competitive Senate contests. Three hypotheses emerge from this model: (1) the greater the diversity of a party's national legislative delegation, the more highly competitive Senate elections we will observe; (2) states in which the ideological heterogeneity of the electorate is relatively high will exhibit a greater number of highly competitive elections; and (3) highly competitive Senate contests will be more common in states with closed primaries than in states with open primaries. We provide strong evidence in support of the first two hypotheses and some evidence in support of the third.