Courting a crucial constituency that had backed him as a Republican, born-again Democrat Arlen Specter assured AFL-CIO delegates that he would champion their interests on issues including health care and union organizing.

Soon after, Mr. Specter's labor credentials were endorsed by President Barack Obama as he began his address to the union officials. Mr. Obama made another powerful contribution to Mr. Specter's re-election campaign later in the day by helping him raise millions at a Philadelphia fundraiser.

Speaking to the labor leaders, Mr. Specter offered a more detailed reprise of remarks he offered last week at the Pittsburgh Labor Day Parade. The veteran senator predicted the passage of a health care reform bill "with a robust public option." He also won a standing ovation with his forecast that a long-sought measure to make it easier to organize unions was on its way to enactment.

Referring to behind-the-scenes negotiations in the Senate, he said, "We have pounded out an employee free choice bill that will meet labor objectives. I believe that before the year is up I will join my colleague Sen. Bob Casey in passing an Employee Free Choice Act that will be totally satisfactory to labor."

Mr. Specter's remarkable political year began with his announcement that he would oppose the Employee Free Choice Act, a measure he had supported in a closely watched procedural vote in the last session of Congress. That was when he was still a Republican. He said then, however, that while he opposed its so-called "card check" provision that would allow workplaces to be organized without a secret ballot election, he did support many of its other key features, notably those designed to ensure speedy contract settlements after organizing drives.

Speaking to reporters after yesterday's convention speech, Mr. Specter said the compromise measure would have enough support to overcome a filibuster attempt, "when the 60th senator is elected from Massachusetts."

Trying to buttress his support, Mr. Specter reminded the delegates that he had backed union positions on a litany of issues through his six terms in the Senate. He cited his tenure as chairman of a key subcommittee with jurisdiction on labor issues and said that he had helped fight back attempts in both Bush administrations to weaken provisions protecting worker health and safety.

"I stood up and fought for labor's interests and won those battles," he said.

As he spoke, Richard Trumka, the soon-to-be president of the labor federation, conspicuously raised his hands in applause behind him.

After serving as one of the opening acts to Mr. Obama's speech to the labor officials, Mr. Specter joined him on Air Force One for a flight across the state to a Philadelphia fundraiser that organizers said would yield close to $2.5 million.

"He was a great senator when he was a Republican and he is going to be an even greater senator now that he is a Democrat," Mr. Obama told the Philadelphia crowd.

Mr. Obama praised Mr. Specter's vote as one of only three Republicans who supported his stimulus package, saying that people who criticize that legislation have "selective memories" about how bad things really were.

"Arlen thought it was more important to answer to the people who sent him to Washington rather than his party," Mr. Obama said.

Majority Leader Harry Reid altered the Senate calendar to accommodate the fundraiser. Mr. Specter has criticized his chief opponent, U.S. Rep. Joe Sestak, for missing votes in order to campaign. Mr. Reid's assistance allows him, at least for a day, to avoid a similar dilemma. Mr. Sestak and another rival in the 2010 Senate race, Republican Pat Toomey, criticized Mr. Specter for the manipulation of the chamber's calendar.

Mr. Specter's former colleague, Rick Santorum, added his derision of his recent ally in a conference call set up by the Republican National Committee.

"I guess it's the price you have to pay the piper to get President Obama to come in. ... He has done a hard left turn to try get into the good graces of this president and obviously he's getting rewarded for that," said Mr. Santorum, who lost a re-election bid in 2006. "The Democratic primary voter in Pennsylvania I think will see through these blatant, pandering flip-flops that he has orchestrated over the last few weeks, and I don't think he'll survive the primary."