The Chamber of Commerce is throwing down the gauntlet against Rep. Justin AmashJustin AmashHouse votes to crack down on undocumented immigrants with gang ties GOP lawmaker taunts House conservatives: Trump’s base is not ‘small faction of obstructionists’ Overnight Finance: GOP plans to unveil tax framework in late September | Critical stretch for Trump tax team | Equifax CEO called to testify | Sanders unveils single-payer bill MORE (R-Mich.), endorsing his primary challenger.

The Chamber is backing businessman Brian Ellis (R) in the race, as it looks to take down the libertarian-leaning congressman.

"The U.S. Chamber is proud to endorse Brian Ellis' congressional campaign to represent Michigan's 3rd congressional district. Both candidates in the primary sought our endorsement, and the choice in support of Ellis on pro-growth issues was clear," Chamber national political director Rob Engstrom said in a statement.

The move comes as no surprise. Amash has long been a thorn in the side of House Republican leadership and pro-business groups, and the Michigan Chamber of Commerce and Grand Rapids Chamber of Commerce had already endorsed Ellis. The national Chamber met with him back in January.

The U.S. Chamber's support is a big financial boon to Ellis in what's quickly turning into a nasty, hard-fought race ahead of the August primary.

It also sets them up for another battle with the fiscally conservative Club for Growth, a group that the Chamber has sparred with throughout the cycle in Republican primaries. The Club supports Amash and has already been on the air backing him in this race.