Party Discipline and Measures of Partisanship

Vote-based measures of partisanship have had a huge impact on the received wisdom about parties in the U.S. Congress. This article employs a cutpoint model to analyze how five such measures respond to changes in preference distributions and to different forms of behavior: partybased discipline and nonpartisan or undisciplined behavior. Three sets of findings are central to ongoing research about parties in legislatures. (1) The well-known and widely used party-voting score cannot discriminate between polar types of behavior. (2) All five measures encourage erroneous inferences of party discipline when only intraparty preference homogeneity may be present. (3) Of the four measures that can discriminate between partisan and nonpartisan behavior, historic congressional averages are often nominally high on a 0-100 scale, however, the averages tend to be closer to no-discipline expectations than to party-discipline expectations. Cumulatively, these findings suggest that, labels notwithstanding, vote-based measures of partisanship are ineffective instruments for detecting genuine partybased voting, party strength, and leadership support.