In the midterm elections of 2010, the Tea Party assault on Republican Party incumbents was successful enough to create within the House Republican Conference a group of insurgents whose uncompromising positions on issues shut down the government, drove a Speaker of the House from office, and opened up an era of even more intense dysfunction in the Congress. Since then, more journalists and scholars have come to realize that to understand politics in a country where the political parties are “big tents,” one needs to appreciate the important role that factional divisions within parties play.

When we began The Primaries Project in 2014, we called congressional primaries “the stepchild of election studies” because journalists and scholars largely ignored them. Since then, however, more and more people have realized that the key to understanding factions in American political parties is to understand congressional primaries. Even though contested congressional primaries are rare and even though incumbent defeats in primaries are even more rare, factional divides within a party manifest themselves in primaries, and the existence of those factions shapes the behavior of members of Congress—even when they are not being directly challenged themselves. Thus, in establishing The Primaries Project four years ago, we set out to understand the dynamics within each political party and the different factions’ impact on the policy agenda—regardless of whether those factions were winners or losers.

This report is divided into six parts below; read it in full by starting with the introduction, or jump to individual sections.