What explains party unity? A test of competing theories

Several theories have been created to explain party unity in Congress, but previous studies have generally assumed the same factors affect party unity for both parties. Given the differences between the two parties (the ideological heterogeneity of their electoral coalitions, how the party's electoral coalitions view partisanship, etc.), this assumption may limit our understanding of the sources of party unity. In this paper, I test three theories of party unity (cartel theory, Conditional Party Government, and Strategic Party Government) on separate panels for the two parties. I find cartel theory helps explain party unity for both parties, but, unlike what has previously been argued, I find this is not entirely due to Reed's Rules. I further find Conditional Party Government better explains party unity for the Republicans, while Strategic Party Government better explains party unity for the Democrats. I provide theoretical reasons for these findings and how they may impact future research.