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Mr. Specter’s party switch is not only costing him existing campaign cash, it could impede future fundraising as Republican contributors turn away and Mr. Specter’s established stable of fundraisers could abandon the candidate, said a Republican campaign adviser.

“A lot of [the campaign staff] are probably as Republican as they are career Specter men,” said the adviser, who did not want to be identified discussing his colleague’s predicament. “Are they going to go work for Toomey or are they going to stay with him?”

Before the party switch, Mr. Specter had raised nearly $1.3 million since January and has $6.7 million in the bank for the 2010 race, according to first quarter campaign finance reports filed this month.

Mr. Specter retains a large pool of donors who are lobbyists and lawyers, groups that have been the financial backbone of his campaigns for decades. He also enjoys broad support among Pennsylvania Democrats.

Richard Ehst, Sovereign Bank’s regional president for southeastern Pennsylvania, said he will not ask for a return of his $250 contribution to Mr. Specter.

“The man has behaved more like a Democrat than a Republican for years,” Mr. Ehst said. “I think moving across the aisle might better define where his allegiances are.”

• Christina Bellantoni contributed to this report.