WASHINGTON — The AFL-CIO, the largest U.S. labor federation, said it will delay endorsing a presidential candidate — though individual unions such as the International Association of Fire Fighters will back a candidate as early as next month.

The AFL-CIO's executive council, meeting in Chicago two weeks ago, decided there isn't enough agreement now among its 55 affiliated members to back a presidential hopeful during the primaries. The federation's rules require a two-thirds consensus.

Still, unions including the Fire Fighters say they will endorse as soon as next month. While union membership in general has been declining, backing from labor, a mainstay supporter of the Democratic Party, could help candidates with money and votes in early primary states such as Iowa and Nevada. The AFL-CIO held a candidate forum in Chicago two weeks ago that its president, John Sweeney, described as "one giant job interview."

"We are aggressively pursuing all of them," Barack Obama's chief strategist, David Axelrod, said when asked about union endorsements. "So is everybody else. The question is how many of them will make an endorsement right now."

The AFL-CIO said its decision doesn't rule out a possible endorsement before the primaries if two-thirds of the unions agree on a candidate.

"It leaves the door open should it develop that there's a consensus," said Karen Ackerman, the AFL-CIO's national political director.

The AFL-CIO lacked agreement to make an endorsement before the Democratic primaries in 2004. At the time, big unions such as the Service Employees International Union backed former Vermont Gov. Howard Dean while the United Steelworkers and 22 others endorsed former House Democratic leader Richard Gephardt of Missouri.

The AFL-CIO says that once it does endorse, it plans its biggest mobilization push ever. "We certainly will have our best campaign effort in the history of the labor movement," Sweeney said last Monday.

Last year, the AFL-CIO spent about $40 million, its most ever for a midterm election, to help Democrats capture control of Congress.

One in four union households voted and about three-fourths of union workers represented by the AFL-CIO supported the candidate endorsed by the federation, according to federation treasurer Richard Trumka. "That's quite a voting bloc," he said.

The Machinists and Aerospace Workers Union, which represents employees at companies such as Boeing, said it plans separate endorsements for a Democratic and Republican candidate before the primaries early next year.

"That's a historic change for us," said Machinists spokesman Richard Sloan. "It will be the first time in 119 years that we've done a dual endorsement."

Thirty-five percent of the union's members are Republicans, Sloan said.