If the unions do delay throwing their weight behind any candidate early in the campaign, it would be particularly frustrating to former Senator John Edwards of North Carolina.

Mr. Edwards has vigorously courted unions, hoping to win the coveted A.F.L.-C.I.O. endorsement and the accompanying organizational backing, especially in the first states to vote.

One-third of Democratic caucus goers in Iowa came from union households in 2004, and one-quarter of voters in the New Hampshire Democratic primary were from union households.

The A.F.L.-C.I.O. is unlikely to endorse Mr. Edwards or anyone else before the primaries, several labor leaders said last week, because unions are so divided over the candidates.

Several unions that like Mr. Edwards are wary of endorsing him because he lags well behind Senators Hillary Rodham Clinton and Barack Obama in the polls.

“There’s a pretty strong sentiment across the labor movement for Edwards,” Steve Rosenthal, a former political director of the A.F.L.-C.I.O., said. “But I think some unions are a little leery of endorsing him without more evidence that he can win.”

Another reason many unions are hesitating to endorse a candidate is their overall happiness with the eight hopefuls. Several back universal health coverage, a major union goal. All have endorsed labor’s main legislative priority, a bill that would make it easier to unionize workers.

“This is a pro-worker field of dreams,” said Bruce Raynor, president of Unite Here, which represents hotel, restaurant and apparel workers. “The field is much better from a worker’s standpoint than it was four years ago.”

Mrs. Clinton and Mr. Obama are wooing unions partly to prevent Mr. Edwards’s securing the A.F.L.-C.I.O. endorsement and partly to pick up individual union endorsements for themselves. Mr. Obama has repeatedly campaigned on behalf of a unionization drive at Resurrection Health Care’s nine facilities around Chicago.

The A.F.L.-C.I.O. endorsement is prized because it opens the door to major union contributions and to support from the federation’s political program, which contacted 13 million members of unions and union households in the 2006 campaign.

“If our board voted today, it would be leaning toward Edwards,” Leo Gerard, president of the United Steelworkers, said. “He showed up at a Goodyear picket line. He just called and said, ‘I’ll be there.’ That kind of stuff really rings home with our members.”

Mr. Gerard said his union would respect the A.F.L.-C.I.O. policy that its 55 member unions not endorse anyone until the federation decided whether it would make an overall endorsement. The Democratic candidates are scheduled to participate in an A.F.L.-C.I.O.-sponsored debate on Aug. 7 in Chicago. The federation said it was expecting 15,000 union members to attend the debate, at Soldier Field.

The president of the federation, John J. Sweeney , said the earliest it would endorse anyone would be late October. Mr. Sweeney said he doubted that any candidate could muster the requisite two-thirds support.

He cautioned against any rush, saying many unions acted too hastily in endorsing Howard Dean and Richard A. Gephardt in 2004, only to see those candidacies flounder early in the primaries.

“There’s merit to delaying, because there are so many good candidates,” Mr. Sweeney said. “We’re seeing a very different attitude this time around. We learned from the last time that we had better be sure that when we endorse we’re supporting the candidate our members want to support. And there’s also an interest in backing a candidate who can win.”

Gerry McEntee, president of the American Federation of State, County and Municipal Employees , acknowledges having learned a lesson in endorsing Mr. Dean.

“We made a big error,” Mr. McEntee said. “The error was that to a large extent it was a leadership decision made without a deep enough effort to see how our members really felt.”

Mr. Edwards and the other candidates are also seeking the endorsement of Change to Win, a rival federation made up of the service employees, the Teamsters and five other unions that left the A.F.L.-C.I.O.

The president of Change to Win, Anna Burger, said her group would probably not make a pre-primary endorsement. Ms. Burger also counseled against any rush to endorse a particular candidate, saying it might discourage other candidates from speaking out on workers’ issues.

“We should give them space” she said.

Andrew L. Stern, the service employees’ president, also suggested that his group was in no hurry to endorse.

“This time, Edwards has done the most to win our support, but Senators Clinton and Obama, they’re not going to let Edwards sneak by them,” Mr. Stern said. “It’s like Roller Derby. We’re waiting for someone to break from the pack. We’re getting closer, but there’s no particular clarity.”

If neither labor federation endorses, that would open the door to individual unions’ endorsements. Union leaders said that the American Federation of Teachers and the Office and Professional Employees International Union were leaning toward Mrs. Clinton and that Unite Here, the Teamsters and the steelworkers were leaning toward Mr. Edwards. A Unite Here endorsement would be a boon in Nevada, because its Las Vegas local has 40,000 members and could dominate that state’s Democratic caucuses.

Two big unions, the service employees and the state, county and municipal employees, might be too torn to endorse anyone. Their locals in Illinois are enthusiastic about Mr. Obama. Several of their locals in New York favor Mrs. Clinton.