Sen. Pat Toomey (R-Pa.) won't pick sides in a Tea Party challenge to Rep. Tim Murphy (R-Pa.), a relief for the longtime congressman.

"No," he said tersely when The Hill asked if he'd endorse Murphy or his primary challenger, former Capital Hill staffer Evan Feinberg (R).

When asked why, Toomey pursed his lips. "I'm just staying out," he said. "It's my decision."

Toomey is beloved by many Pennsylvania conservatives for challenging Sen. Arlen Specter (R-Pa.) in 2004 and again in 2010, forcing Specter to leave the GOP.

An endorsement from Toomey would have been a huge score for Feinberg, giving him the state's most popular Republican and one of Washington's most powerful conservatives in one fell swoop.

Many of Toomey's allies are backing Feinberg: His support list includes his two former bosses, Sens. Rand Paul (R-Ky.) and Tom Coburn (R-Okla.). The deep-pocketed, fiscally conservative Club for Growth, which Toomey headed until 2009, has already run TV ads slamming Murphy. Both Paul and Coburn compared Murphy to Specter in their endorsements.

Murphy released an internal poll in late February touting a 60-point lead in the primary, and Feinberg had only $40,000 in the bank as of last election. But if the Club for Growth decides to seriously contest the race and local conservatives get on board Murphy could face a tough race.

- This post was updated at 6:58 p.m.