If you need more evidence of Americans Elect’s claim that party primaries are an increasingly effective means of maintaining party discipline, here’s Charlie Cook:

The Cook Political Report counts 22 House Republicans (not including member–versus-member races) with primaries worth watching. Some are more ideological or tea party-infused (Rep. Fred Upton’s race in Michigan); some are more generational or anti-entrenchment (Rep. Ralph Hall’s contest in Texas); and some are predominantly redistricting-related (Rep. Paul Gosar’s fight in Arizona).[...]

In the postwar era, no election has seen more House incumbents lose primaries than general elections. But with 11 member-versus-member primaries on tap and a bevy of other interesting primaries brewing, particularly on the GOP side of the electorate, such an outcome isn’t out of the question.