Former Rep. Mark Neumann (R-Wis.) has landed yet another major conservative backer in his primary race against former Wisconsin Gov. Tommy Thompson (R): Sen. Tom Coburn (R-Okla.) will appear at a Washington fundraiser for Neumann.

Freshman Sen. Rand Paul (R-Ky.), a Tea Party favorite, endorsed Neumann Thursday, and the deep-pocketed, fiscally conservative Club for Growth has thrown its support behind him and against Thompson as well.

Thompson has been criticized by many conservatives for his early backing of President Obamas calls to reform the health insurance system, although he backed away from endorsing the final bill.

Neumann, who ran in the 2010 GOP gubernatorial primary, has sought to cement his conservative bona fides, and this endorsement is the latest step toward that goal.

Also in the GOP race is State Assembly Speaker Jeff Fitzgerald, another conservative.

The likely Democratic nominee is Rep. Tammy Baldwin.

They are running for retiring Sen. Herb Kohls (D-Wis.) seat.