Seniority, Leadership Selection, and Party Cohesion in the Romania and US House of Representatives

Party cohesion is key to understanding legislative politics. Existing studies of party cohesion tend to focus on electoral institutions, ignoring legislative party organization that can create separate effects. This study focuses on how and to what extent seniority affects party cohesion. We begin with the assumption that individual party members have heterogeneous preferences, so that some are more distant from the party center than others. More ideologically distant members exert high demands to move the party platform toward their own ideal points, while those who are ideologically proximate low-demanders accept the party position as it is. We argue that it is in the collective best interest of party members to select leaders who will be reliable agents. Where advancement to leadership is not purely a function of length of service, by contrast, there are no longer benefits of expressing loyalty for ambitious senior members. In effect, individual members have little incentive to toe the party line. We thus expect to see an inverse relationship between the seniority norm and party cohesion. Applying the Bayesian procedure for estimation and inference for spatial models of roll call voting (see Clinton, Jackman, and Rivers 2004), we estimate individual legislators’ ideal points and compute party positions by including each party as an individual in calculating roll-call scores (see Cox, Heller, and McCubbins 2008). We test our hypothesis utilizing data primarily from roll-call votes and legislative assignments in the Romanian Camera Deputatilor and the US House of Representatives for the period of 2006-2010 and 1994-2010.