Club for Growth, which bundles contributions for the free-market conservatives it endorses and runs ads on their behalf, is supporting Simpson's primary opponent.
Another group, the Senate Conservatives Fund started by former GOP Sen. and now Heritage Foundation President Jim DeMint, is raising money to run ads against McConnell and Sens. Thad Cochran, R-Miss., and Pat Roberts, R-Kan., each of whom is facing a tea party primary challenge.
McConnell orchestrated a boycott of a consulting firm that does business for the Senate Conservatives Fund after the group endorsed his primary opponent, businessman Matt Bevin.
Last week, the U.S. Chamber of Commerce launched an ad backing Simpson, an eight-term Republican.
The Chamber's involvement marked the second time in recent months that the group has taken the side of GOP establishment against conservative activists. The group backed Bradley Byrne over tea party favorite Dean Young in a special congressional runoff primary in Alabama, pumping at least $200,000 into the race. Byrne won the Nov. 5 contest.
Simpson's primary challenger in Idaho, Bryan Smith, is being supported by the Club for Growth, a group that has helped topple Republican incumbents in past primaries, including longtime Sen. Richard Lugar of Indiana. But Democrat Joe Donnelly ended up winning Lugar's seat in the general election, underscoring one of the complaints by mainstream Republicans — that hewing too far to the right is costing elections.
Conservative groups have taken on outsized importance as more GOP House members represent solidly Republican districts in which the only real threat the GOP feels is from tea party candidates running to their right.
The groups' status in statewide Senate races is a little different. Club for Growth and Senate Conservatives Fund on several instances have won the battle but caused Republicans to lose the war.