Former Rep. Pat Toomey (R-Pa.), whom Specter defeated by less than two percentage points in 2004, said he hasn't ruled out a 2010 rematch, but has no timetable for a decision.
Toomey, the president of the conservative Club for Growth, insisted Specter faces more hurdles than he did four years ago.
"He's significantly more vulnerable now than he was in 2004," Toomey said in an interview.
He argued that Specter's core constituency in the GOP, which he called "liberal and moderate Republicans," have since left the party and will be unable to vote in Pennsylvania's closed Republican primary. That will make it more difficult for Specter to prevail against a conservative opponent, Toomey said.