A friendship forged on Amtrak was reinforced on Air Force Two yesterday as Vice President Joe Biden flew into Pittsburgh, urging a union crowd to rally to the re-election of Sen. Arlen Specter.

Under a steady rain, state AFL-CIO President Bill George along with Jack Shea, president of the Allegheny County Labor Council, greeted the senior Democrats as they addressed a kick-off rally for the city's annual Labor Day Parade.

Beads of rain glinted on a TelePrompTer on the makeshift stage as the former Senate colleagues declared their allegiance to chief issues on labor's legislative agenda. A crowd of several hundred listened to their brief speeches while a much larger throng -- either intent on parade preparations or intimidated by the Secret Service magnetometers -- massed in the upper lots of the Mellon Arena before the opening of the Downtown march.

"I'm not sure you're going to invite me to many more Labor Days," Mr. Biden said under the soggy skies. "Last time I was here I had to leave because [of] a hurricane," he said, referring to last September's Hurricane Gustav, "... and today it's raining, and if you all are foolish enough to listen to me in the rain, I have real grave doubts about your judgment."

Mr. Biden repeated the tale of his campaign to woo Mr. Specter to the Democratic Party conducted while they shared the commute from Washington on Amtrak.

"I've been beating on him for 15 years to switch," the vice president said. "He did and I'm proud."

Mr. Specter shared the reminiscence.

"Been riding Amtrak with a good-looking young fellow who used to be a mere United States senator," he said. "He called me up enough that I reverted to my roots to become a Democrat."

By his own acknowledgement, however, the veteran senator's stunning party switch earlier this year had more to do with a strong GOP challenge from Pat Toomey than his friend's powers of persuasion. Mr. Specter avoided one primary fight but now faces another as he courts the nomination of his new party in the face of opposition from state Rep. Bill Kortz of West Mifflin and Rep. Joe Sestak of Delaware County.

In his brief speech, Mr. Specter underscored his union ties, predicting swift action on a bill designed to make workplace organizing easier and saying of the highly charged health care debate, "At a minimum we need a robust public option and we're going to get it."

He won a smattering of applause as he predicted that President Barack Obama would endorse a closely watched International Trade Commission ruling that would levy penalties on China for tire exports to the United States deemed unfair by panel.

"I'm optimistic that President Obama will uphold that ruling and save thousands of jobs for steelworkers," he said.

Continuing the show of administration support for the incumbent, Mr. Biden and Mr. Specter left the rally to take another Air Force Two flight across the state for an appearance in the vote-rich Philadelphia suburbs, in Chester, Delaware County, just outside Mr. Sestak's district.

While they were on their way, his rivals for the nomination worked the crowd gathered for the parade. As they marched, the energetic Mr. Sestak zigzagged back and forth between the curbs lining the route, seemingly intent on shaking every hand in the rain-thinned crowds.

Mr. Specter's early poll numbers, while soft enough to suggest some vulnerability, have still been running well ahead of those of Mr. Sestak.

"This is part of the effort I need to make to show people there is a credible alternative," the congressman said of his parade performance. He shrugged off the incumbent's administration backing, as he said of the marchers and spectators, "Those are the people I need."