“It’s the party-switching,” said a senior Democrat close to the White House who has tried to help Specter but now sees his prospects dimming. “He just didn’t finesse it and wound up looking pretty ham-handedly like he was trying to save his own ass.”

President Obama has raised money for Specter and is featured praising him in a new ad the senator’s campaign began airing Tuesday.

But a well-placed Democrat said unambiguously that Obama would not be returning to Pennsylvania to campaign for Specter before the May 18 primary…

Without an 11th-hour visit from the president, it could be hard for Specter to drive turnout in Philadelphia, his hometown, where many of the Democratic primary votes come from and where the senator must perform well to win.