The Transport Workers Union, which represents 140,000 active workers, is expected to announce Friday that it will become the first union that had backed John Edwards to switch its endorsement to Barack Obama, one union adviser said Thursday night. The union, which endorsed Mr. Edwards, last September represents transit workers in New York City, flight attendants at Southwest Airlines and ground crew for American Airlines.

The Transport Workers Union is especially strong in New York State because the union’s New York City branch, Local 100, with more than 30,000 members, is one of the city’s largest and most vocal unions. The union, which is based in Manhattan also claims 60,000 retirees nationwide as members.

Labor leaders say many of the unions that had endorsed Mr. Edwards, including the United Steelworkers and the United Mine Workers, are waiting for the race to clarify before they decide on another endorsement. Union leaders say that it is awkward to endorse one candidate who falls by the wayside, and even more embarrassing to endorse a second candidate, and that

have that candidate also fail to get the nomination.



Richard Trumka, the A.F.L.-C.I.O.’s secretary-treasurer, said on Thursday that he did not see the labor federation–a grouping of 55 unions representing 10 million workers– making an endorsement in the near future.

He said that at the moment neither Hillary Rodham Clinton nor Barack Obama is in a position to muster the necessary two-thirds support to secure an endorsement.

“We think they’re both very good candidates,” he said. “They’d both be a vast improvement over the person now in the White House.”

Union leaders say that if Mrs. Clinton or Mr. Obama develop a commanding lead in the delegate count, the A.F.L.-C.I.O. might seek to move soon afterward to endorse the front-runner.