The Republican Main Street Partnership PAC announced Thursday it has launched a 60-second radio spot featuring GOP Rep. Fred Upton's endorsement of former state Sen. John (Joe) Schwarz's bid to succeed retiring Rep. Nick Smith, R-Mich.

"I've known Joe Schwarz a long time and he's always put policy over politics," Upton says in the ad. "Sometimes politicians don't like to tell the truth. Joe Schwarz has always been a straight shooter, both as a doctor and a public servant."

Upton's endorsement of Schwarz puts him at odds with his fellow Michigan colleague, Smith, who is backing his son, attorney Brad Smith, as his successor.

Other Republicans running for the GOP-leaning seat include state Reps. Clark Bisbee, and Gene DeRossett, and former state Reps. Paul DeWeese and Tim Walberg.

The PAC, which supports the election of GOP moderates to Congress, plans to run the ad on two stations during weekdays between now and the Aug. 3 primary, a PAC spokeswoman confirmed.