It seems either a cruel or fitting twist. Senator Arlen Specter left the Republican Party last year and became a Democrat to save his political career. Now, at 79, he suddenly finds that the party he switched into may not provide a safe haven.

The stunning upset by a Republican Senate candidate last week in Massachusetts has filled Democrats across the country with dread, forcing some into retirement and prompting more Republicans to enter races they might have sat out. And it has invigorated Mr. Specter’s rivals here on the left and right as they attack him for having been aligned with policies of both the Bush and Obama administrations.

Representative Joe Sestak, a second-term Democrat who is backed by progressives and who is challenging Mr. Specter in the May primary, has driven Mr. Specter to the left.

If Mr. Specter wins the primary, he will have to tack back to the center to face former Representative Patrick J. Toomey, the Republican Senate candidate who six years ago narrowly lost the Republican Party nomination to Mr. Specter.

It was Mr. Toomey’s strength in the polls last year that forced Mr. Specter to switch parties in the first place. As a former president of the Club for Growth, a conservative activist group, Mr. Toomey said he expected to rally enthusiasts from the Tea Party movement, which bolstered Scott P. Brown, the Republican who won in Massachusetts.

“And Pennsylvania is a lot more conservative than Massachusetts,” Mr. Toomey said.

Mr. Specter acknowledged in an interview even before the Massachusetts election that the national mood could work against him. But he also expressed enormous confidence in his ability to survive. Other Democrats who are retiring, he said, are doing so out of fear of losing — they had not actually been beaten but were merely discouraged.

“I am not discouraged,” he declared.

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