Pennsylvania's top labor union, the AFL-CIO, has thrown its support behind incumbent U.S. Sen. Arlen Specter, meaning challenger Joseph Hoeffel is without the backing of a typically reliable ally of Democratic candidates.

That the state chapter of the AFL-CIO lent its support to Specter, a moderate Republican, doesn't qualify as a political surprise -- it did so in Specter's last general election race against Democrat Bill Lloyd.

The AFL-CIO also endorsed Specter during his nail-biting primary run against Republican challenger Pat Toomey earlier this year.

Many labor officials interviewed since the primary said they thought the best Hoeffel could hope for was a neutral stance from the AFL-CIO, as was the case in 1992 when Specter defeated Democrat Lynn Yeakel.

"Arlen has 24 years of relationships with the state leaders [of the AFL-CIO]," said Hoeffel, a congressman from northeast Philadelphia.

The AFL-CIO, 900,000 members strong, made its official announcement yesterday, following an executive council meeting in Hershey. The 58-member executive council gave Specter, a four-term incumbent, the two-thirds vote he needed to secure the union's endorsement.

Specter was the only Republican candidate on the list of 16 endorsements for congressional and state-level row offices.

"I'm especially heartened by the fact that no Democrat in modern times has won a statewide office without [the] AFL-CIO endorsement, so my winning it is a very good omen for Election Day," Specter said in a statement.

Because of the union's size, the AFL-CIO stamp of approval is a coveted prize for anyone seeking state or congressional office.

But rank and file don't always vote with the top union brass. In 2002, then gubernatorial candidate Ed Rendell swept to a primary victory over Auditor General Robert P. Casey even though labor officials, still peeved at Rendell for demanding labor concessions during the Philadelphia budget crisis, pledged their support to Casey.

Still, Specter's AFl-CIO endorsement can't be welcome news for his challenger. Recent polls show that Specter holds a big lead over Hoeffel, 54 percent to 36 percent among all registered voters and 56 percent to 36 percent among likely voters.

Hoeffel put on a brave face yesterday, down-playing the significance of the AFL-CIO decision to cross party lines.

"Labor households are going to vote Democratic this year," he said.

He also said he hoped to reap the benefits of a John Kerry victory in Pennsylvania.

The AFL-CIO's list of endorsements for the state Legislature will be released today. The union's national body has already endorsed Sens. John Kerry and John Edwards in the presidential race.