From Brookings:

Primaries are now a major political pressure point for insurgents and activists, who are more ideological and extreme than the public as a whole. Most incumbents win their primaries, but the mere threat of being “primaried” can make legislators wary of casting votes or accepting compromises that might anger activists. “Members…work to avoid acting in ways that could lead advocacy groups to oppose them in the future,” write our Brookings colleague Elaine C. Kamarck and the R Street Institute’s James Wallner.