The American Conservative Union's political action committee endorsed Rep. Don Manzullo (R-Ill.) over Rep. Adam Kinzinger (R-Ill.) Tuesday morning, making it the latest conservative group to jump in for Manzullo in the member-vs.-member primary.

The group joins the Tea Party-affiliated Freedomworks, the anti-abortion-rights Eagle Forum and the 60 Plus Association, a conservative seniors group, in supporting the longtime congressman.

Kinzinger has the backing of House Majority Leader Eric Cantor (R-Va.) as well as fellow Illinois Republican Reps. Aaron Schock and John Shimkus. The Campaign for Primary Accountability, an anti-incumbent super-PAC, is also backing him.

The race has turned into a proxy war between conservative groups and Republican House leaders. Kinzinger has received donations from House Majority Whip Kevin McCarthy (R-Calif.) and House Speaker John Boehner (R-Ohio) as well, although neither has endorsed in the race and Boehner has declared he is neutral.

Kinzinger decided to run against Manzullo in a new district encompassing part of both their old districts after his district was dismantled by a Democratic gerrymander. The two will face off in a primary next Tuesday.