A centrist Republican group is joining with former senator Olympia Snowe (R-Maine) in an effort to raise and spend $8 million to defend centrist GOP incumbents in 2014 congressional primaries.

Main Street Advocacy and its allied super PAC, Defending Main Street, will take a step forward after playing a minimal role in recent elections. The move comes in direct response to the growing influence of conservative outside groups like the Club for Growth, which the head of Main Street denounced Tuesday as a "cancer" on the GOP.