Third-party threat and the dimensionality of major-party roll call voting

This paper assesses the influence of the electoral threat of third parties on major-party roll call voting in the US House. Although low-dimensionality of voting is a feature of strong two-party politics, which describes the contemporary era, there is significant variation across members. I hypothesize that major-party incumbents in districts under a high threat from third-party House candidates cast votes that do not fit neatly onto the dominant ideological dimension. This hypothesis is driven by (1) third party interests in orthogonal issues, and (2) incumbents accounting for those interests when casting votes in order to minimize the impact of third parties. An empirical test using data from the 105th to 109th Congresses provides evidence of this effect.