Leadership power, preference homogeneity, and legislative party conflict*

Over the past few decades, there has been a considerable increase in the level of partisan conflict. What explains the extent of partisan conflict experienced in legislatures? To examine this question, we develop aggregate-level measures of inter-party conflict from roll-call voting data on procedural and final passage votes during the 1999-2000 and 2003-2004 sessions to examine the mechanisms of polarisation in U.S. state legislatures. We systematically model inter-party conflict as a function of institutional rules and preferences. Our results suggest that the level of inter-party conflict (i.e. party polarisation) in legislative voting is largely related to the homogeneity of members' electorally-induced preferences. However, we also uncover interesting institutional effects. Although minority party rights is the only institutional variable associated with inter-party conflict on final passage votes, majority party leaders' committee appointment rights, committees' discretion in reporting bills to the floor, and minority party rights each are strongly associated with inter-party conflict on procedural roll-call votes.