Rand Paul backs primary challenge to Rep. Tim Murphy

Both men told The Hill in December that they might endorse Feinberg.

Paul described Feinberg as "a tireless advocate for limited government and individual liberty."

"During his time in my office, I learned from personal experience that Evan has the skills, experience and philosophy necessary to help fix what's wrong with our country," he said in a statement. "The only way to get our fiscal house in order and restore the American Dream is to replace self-interested politicians with candidates who will fight for principle and will put the next generation before the next election."

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 he has so far struggled to raise much money, with only $40,000 in the bank at the end of the year.

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.