Sen. Arlen Specter (R., Pa.) said yesterday that he would oppose legislation making it easier for workers to form unions, dealing a severe blow to organized labor's top political priority as he faces a 2010 primary challenge from the right.

Union leaders were counting on Specter to be the 60th vote needed to stop an expected GOP filibuster of the Employee Free Choice Act later this year. He was the lone Senate Republican to support consideration of the measure in 2007, when it stalled in the Senate.

"It is a very emotional issue, with labor looking to this legislation to reverse the steep decline in union membership, and business expressing great concern about added costs which would drive more companies out of business or overseas," Specter said in a Senate floor speech yesterday.

Specter made his announcement as a likely primary challenge looms from former Rep. Pat Toomey, head of the conservative Club for Growth, who came within 2 percentage points of beating Specter in 2004. Toomey called Specter's decision a "flip-flop."

Specter said he was concerned that the bill would eliminate union-organizing elections by secret ballot, "the cornerstone of how contests are decided in a democratic society." And a recession is the wrong time to increase business costs, he said.

Business groups have already spent about $20 million lobbying against the legislation they call "card check," which has inspired fierce grassroots campaigning on both sides. Pressure was perhaps most intense on Specter because of his position as a swing vote. Easy House passage of the bill has been expected.

GOP conservatives were already furious with Specter over his crucial vote for President Obama's $787 billion stimulus last month. Until that firestorm, Toomey had been expected to run for governor instead of seeking a rematch.

"It's nice to see Sen. Specter reverse his position in a positive direction on card check, but I wish it didn't take primary opposition to get him to do it," Toomey said in a statement.

Pennsylvania political analysts said Specter avoided fanning the flames from the stimulus vote with yesterday's move.

"Politically it was the only thing to do," Republican consultant Charles Gerow said. "The only place Arlen Specter would be vulnerable is in the primary, if he had the social conservatives and the business conservatives united against him. This will help bring the economic conservatives back. Is he out of the woods? No, but he took a giant step in the right direction."

The bill, which Obama has favored, would give workers the right to organize as soon as a majority of those in a workplace sign cards agreeing to representation. Under current law, employers can demand a secret ballot, which unions say opens workers to intimidation. The proposal also includes binding-arbitration provisions.

Business groups praised Specter, while labor leaders accused him of betraying workers and vowed to press on.

Senate Majority Leader Harry Reid (D., Nev.) said the bill would be brought back in some form, saying: "Anyone who thinks they're burying card check because of Specter's statement in an effort to avoid a primary in Pennsylvania should not think this legislation is going to go away."

Still, even before Specter's announcement, it appeared unlikely that sponsors could raise the 60 Senate votes to avert a filibuster. Democrats control the chamber by 58-41, with one seat still unsettled, and some moderate Democrats have expressed deep reservations about the bill, including Blanche Lincoln and David Pryor of Arkansas and Mary Landrieu of Louisiana.

John Engler, president of the National Association of Manufacturers, praised Specter for opposing legislation that would "destroy jobs and make our country less competitive."

AFL-CIO president John Sweeney called Specter's position "frankly, a disappointment and a rebuke to working people, to his own constituents in Pennsylvania and working families around the country."

Andy Stern, president of the Service Employees International Union, vowed to keep pressing for labor-law changes.

In recent weeks, labor representatives said they had offered Specter election help - even to the point of registering members as Republicans so they could vote in next year's primary - if he would support the bill.

The implication that his vote was for sale irked Specter, and he referred to it yesterday, saying: "This announcement should end the rumor mill that I have made some deal for my political advantage. I have not traded my vote in the past, and I would not do so now."

Although he agreed with business' critique of the bill, Specter also said labor laws needed an overhaul. He suggested new requirements that certification elections be held in a timely manner, and that the government guarantee unions access to worksites during the run up to those elections so they could campaign on an equal footing with management.

But National Right to Work, an anti-union group, said, "His proposals are totally unacceptable, as they will be used by Big Labor to corral more workers into forced unionism."

Specter and his staff made courtesy calls yesterday to discuss his decision, according to area union officials, including Wayne MacManiman, Philadelphia-based district chairman of Local 32BJ of the SEIU. The local represents office cleaners and other building maintenance personnel.

"It is disappointing," MacManiman said. ". . . I doubt it will be the end of it. . . but Specter was kind of the horse everyone was counting on."

Specter staffers also notified Tony Forte, political director of the regional United Auto Workers union, based in Fort Washington.

"I can't see labor supporting him in any way, shape, or form," Forte said. Although Specter voted in 2007 to at least let the bill come to a vote, Forte said, "this time he's certainly under the microscope from his party. He had to come out showing he wasn't in labor's pocket."

Contact staff writer Thomas Fitzgerald at 215-854-2718 or tfitzgerald@phillynews.com.