The idea that primary elections are causing polarization because voters in primaries prefer extremism is something maintained by a vast range of pundits and politicians, as well as more than a few scholars. But as Mike Cowburn shows in Party Transformation in Congressional Primaries, it's not remotely that simple. This book brings an impressive array of evidence to examine what exactly primaries are and how they are affecting the U.S. political system today.

At the core of the book is a key distinction between factional primaries (ie, candidates receiving support from very distinct groups and elites within the party) and ideological primaries (ie, candidates describing the contest in ideological terms and fights over the direction of the party). The book demonstrates that both these types of primaries have significantly increased in frequency over the past two decades, changing the nature of primaries and of the parties.

In Cowburn's measurements, there is some overlap between ideological and factional primaries, and a good number fall into both categories. This invites questions about the nature of the distinction. For example, when House Republican Leader Eric Cantor lost to a challenger in a 2014 Republican primary, which was a more important signal to other Republican officeholders: that Cantor was challenged from the right by a