What Explains Party Unity? Evidence from U.S. State Legislatures

Party unity is central to studying legislative politics and explaining the organization of legislative chambers, passage of policy, and even electoral outcomes. Its effect has been shown at the subnational, national, and international levels. While party unity has been used to explain many phenomena in legislative politics, there is much less known about the determinants of party unity, especially at the subnational level. We leverage a dataset of party unity in 95 U.S. state legislative chambers from 2002 to 2014, to test the impact of party structures, institutional rules, and contextual factors on the level of party unity. We find that majority status, party seat share, and the other party's unity impact party unity similarly for both parties, but the effect of inter-party heterogeneity differs by party. These findings offer a new and unique insight to how partisan theories operate differently within state legislative versus Congressional parties.