Party composition in the American states: Clarifying concepts and explaining changes in partisanship since the 1950s

Students of state politics have long been interested in the partisanship and the degree of interparty competition in the American states. This has led to numerous attempts to quantify state-level partisanship and competition, the most significant by Ranney (1965). Interestingly, however, scholars have never specified clearly the relationship between these two concepts as measured by the Ranney index. This research attempts to do just that and reveals that the two are different measures which are related systematically. More important, understanding changes in a state's partisanship is determined to be crucial to understanding changes in its level of two-party competition. As such, in order to explain changes in partisanship and competition across the American states from the 1950s to the 1970s, we focus on accounting for changes in partisanship. To this end, changes in aggregate demographic variables account very well for states' movement along the Ranney index over the last 3 decades.