His message: that if Democrats vote against him today, the Tea Party will take over Pennsylvania.

"They want to eliminate the EPA, they want to go back to the gold standard," Specter told Mitchell as it rained on the street behind them, and as Philadelphians came up to the glass to snap photos of the 80-year-old senator.

"If you don't field the strongest candidate," Specter warned, "they're going to take over."

Whoever wins today's primary will go on to face Republican Pat Toomey, a former congressman, in November. Toomey is a favorite of some Tea Partiers, as the Dick-Armey-led group FreedomWorks will look to coordinate its supporters (many of whom are Tea Partiers) on his behalf. Toomey is also getting help from the DC-based free-market conservative group he used to run, the influential Club for Growth.

Specter warned repeatedly that he is the only candidate who can defeat Toomey, which he did once before, in his 2004 reelection race. Sestak, meanwhile, actually polls better against Toomey: the Republican beats both candidates in a close race, according to recent polls, but he bests Specter by about 5 percentage points more.

Specter's argument: he's beaten Toomey before, and he's the only candidate with enough skill and vigor to do it again.

"The object is not Arlen Specter or Sestak," Specter told Mitchell. "Beating the Tea Party gang is more important than who does the beating."