Joe Torsella, former head of the National Constitution Center in Philadelphia, pulled out of the race Thursday, saying Specter's switch from the Republican to the Democratic party "transformed this election," Congressional Quarterly Politics reported Friday.
The campaign "probably would be negative, personal and more about Senator Specter's past than about our common future, and that won't do Pennsylvanians any good," Torsella said.
"No one asked me to take this step, and I haven't asked for or been offered anything to do it," Torsella said.
He said he would refund contributions to his Senate campaign. Torsella reported raising $597,000 in the first quarter.
Pennsylvania state Rep. Josh Shapiro also withdrew from the Democratic Senate race after Specter's party switch, CQ Politics said. However, Democratic state Rep. William C. Kortz II filed papers for his senate candidacy last week with the Federal Election Commission.