Sen. Tom Coburn (R-Okla.) is backing one of his former staffers in a primary battle against Rep. Tim Murphy (R-Pa.).

Coburn, a favorite of conservatives who hinted to The Hill that he might back Evan Feinberg in the race in December, announced his decision on Monday.

"I strongly support Evan Feinberg for the Republican nomination in Pennsylvania's 18th congressional district," Coburn said in a statement. After saying Feinberg had, in his three years on Coburn's staff, helped him design a "free-market alternative" to Democrats' health insurance overhaul, he praised the young candidate's abilities.

"Evan is the fresh face the conservative movement needs to broaden its appeal," Coburn writes. "He possesses the vigor and determination necessary to wage the war against entrenched special interests in Washington."

Feinberg has generated a lot of attention from Washington conservatives: FreedomWorks has endorsed him already, and the deep-pocketed Club for Growth has been hitting Murphy as a "liberal" in television ads. But Feinberg, who also worked for Sen. Rand Paul (R-Ky.), has so far struggled to raise much money, with only $40,000 in the bank at the end of the year.

Coburn called Murphy, a longtime incumbent, an "Arlen Specter Republican," comparing him to the centrist former senator who left the GOP because of a primary challenge. 

"Just like other career politicians, Congressman Murphy says he is for cutting wasteful spending but then votes to waste your tax dollars on wasteful pork projects," he said. "He even voted for the Bridge to Nowhere [earmark], just like Arlen Specter."

The two are squaring off in a Republican-leaning congressional district outside Pittsburgh. Democrats are hopeful that they can make the race a competitive one if the more conservative Feinberg wins the primary.