Courting the support of 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.

In a more detailed reprise of remarks he offered 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 compromise 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 was followed by President Barack Obama, who opened his remarks by praising Mr. Specter to the delegates.

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 the state AFL-CIO convention earlier this summer, he insisted that the shift was not a concession to his new party or his endangered re-election prospects. Rather, he argues, it is the bill that has changed, not his essential position.

Speaking to reporters after today'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.''

"I do not believe there will be any Republicans [voting yes],'' he said.

He acknowledged that some Senate Democrats might also oppose the organizing bill on final passage but suggested that the caucus would be unanimous in fighting back a likely filibuster.

Trying to buttress his support, Mr. Specter reminded the delegates that he had supported 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.

Mr. Specter was one of the opening acts to President Obama's speech to the labor officials. Mr. Specter and the president were to fly to Philadelphia later in the afternoon, where Mr. Obama was to join the embattled incumbent, headlining a fund-raising event expected to yield as much as $2.5 million. Majority Leader Harry Reid altered the Senate calendar to accommodate the fund-raiser. Mr. Specter has criticized his chief opponent, 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.

"While it's nice to have the president's support, while it's nice to have Vice President [Joseph] Biden with me at the Pittsburgh Labor Day Parade,'' Mr. Specter said in a final appeal as he concluded his remarks, "I realize elections are won or lost with the support of the AFL-CIO and I again thank you for your help in the past and I will do my utmost to merit your support in the future.''