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Mr. Toomey has also enjoyed support from the Club for Growth, where he previously served as president, as well as from the U.S. Chamber of Commerce and American Crossroads GPS, the group headed by Karl Rove, the mastermind behind George W. Bush’s electoral successes.

Meanwhile, Richard Bloomingdale, head of the AFL-CIO, which has more than 850,000 members, said he is “starting to see people come alive in central Pennsylvania” as more people have volunteered to work the local phone banks and go door-to-door on behalf of Mr. Sestak.

“It’s coming to life,” he said. “It took a little bit longer, and we are probably a little bit behind where we were in 2008, but I’m starting to feel more enthusiasm.”

He attributed some of the renewed energy to Christine O’Donnell, the “tea party”-backed candidate running for the Senate in neighboring Delaware. Ms. O’Donnell has run a campaign commercial that aired in the Philadelphia television market, which also serves Delaware, in which she assures voters that “I’m not a witch.” That followed the airing of an 11-year-old TV sound bite in which she admitted to “dabbling” in witchcraft while in high school.

“That was a real boon for us,” he said. “Maybe people were starting to believe we were drifting too much to the left, and they wanted to bring it back. But then they see people like her, and it’s like, ‘Wow, maybe these people aren’t too far out, but the other side is.’ “