Sen. Arlen Specter won the backing of the state's largest labor organization yesterday, with the Pennsylvania AFL-CIO endorsing him over Rep. Joe Sestak in the May Democratic primary.

It was a powerful affirmation of how smoothly Specter has managed his political transition to the Democratic Party since leaving the Republican Party less than a year ago.

"He is a proven leader who has stood with working families when the chips are down," said Bill George, president of the state AFL-CIO, which represents 900,000 workers. George cited Specter's crucial role last year in passing the $787 billion federal stimulus measure, which he said "prevented this nation from sinking into another Great Depression."

Specter was one of only three Republicans to support the legislation, and his vote forced him to abandon the GOP when the party base rebelled. After former Rep. Pat Toomey announced a primary challenge, Specter became a Democrat, saying he could not win renomination as a Republican.

Sestak noted yesterday that he has a 100 percent rating from the national AFL-CIO for his voting record during two terms in Congress, while Specter's career rating is 61 percent. He attacked Specter for voting for the economic policies of President George W. Bush while a Republican senator.

"Pennsylvania workers need a senator they can count on to be there when they need him, not only when he needs them during an election," Sestak said.

Several union locals have backed Sestak, including the politically active United Food and Commercial Workers, but Specter has racked up all of the major labor endorsements - including the Pennsylvania State Education Association, which represents 200,000 teachers.

Labor is an important source of cash and muscle in Democratic politics. Specter also has received the endorsement of the Democratic State Committee and hundreds of rank-and-file Democratic officeholders and activists - not to mention the White House and Gov. Rendell.

It takes a two-thirds vote of the AFL-CIO's executive committee to win an endorsement, and Specter easily passed that with a vote of 34-9.

Specter has received AFL-CIO support before, notably in 2004, when the group backed him over then-Democratic Senate nominee Joe Hoeffel.

The group also made endorsements in competitive Democratic primaries for the House yesterday, including Doug Pike in the Sixth District and State Rep. Bryan Lentz in the Seventh District.

There was no endorsement in the governor's race, with four Democrats dividing labor loyalties. The AFL-CIO will consider making an endorsement at its convention in Pittsburgh beginning April 11, George said. He added that none of the candidates could muster two-thirds of the executive committee at this point.

As a Republican, Specter came out against labor's top legislative priority, a bill that would make it easier to unionize workplaces by providing a mechanism to bypass secret-ballot elections. He since has advocated a compromise that would preserve the secret vote but limit employers' use of delaying tactics to stop unionization.

George said that Specter's AFL-CIO rating since becoming a Democrat is 84 percent. But he said Specter had done things that go well beyond floor votes over the years, such as advocating for steelworkers in trade disputes with China and securing money for job-creating projects such as Delaware River dredging and the repair of river locks and dams in the Pittsburgh area.

"Sestak's been there just four years," George said. "He's a good guy and people liked him, but they really doubt his ability to raise money and beat Toomey," the likely GOP nominee. "In the atmosphere of today, we're looking for somebody with experience who can take on some of the right-wing nuts in debate," George said.