CHICAGO – As the AFL-CIO opened its 50th anniversary convention here yesterday, two of America’s largest unions, the Service Employees International Union and the International Brotherhood of Teamsters, withdrew from the national labor federation.

The move, which was expected, came as the labor movement engaged in a fractious public debate over how to reverse two trends: declining membership and waning political influence.

“What was being done at the AFLCIO is not working. We’re going to do something new,” the president of the Teamsters, James Hoffa Jr., said. “We do not take this action lightly. We realize it will have a negative effect to some extent on the AFL-CIO.”

“Our goal is not to divide the labor movement, but to rebuild it,” one of the most stalwart critics of the federation, the Service Employees’ president, Andrew Stern, said. “This was not a happy or an easy decision. In itself, it does not represent an accomplishment but simply an enormous opportunity.”

The president of the AFL-CIO, John Sweeney, was not available for comment following the announcement that the two unions were leaving the federation. In a speech earlier yesterday to the federation’s convention, he called a related decision to boycott the gathering “a grievous insult” that will make it even harder for the struggling labor body to fight on behalf of workers.

“It is a tragedy for working people because at a time when our corporate and conservative adversaries have created the most powerful anti-worker political machine in the history of our country, a divided movement hurts the hopes of working families for a better life,” Mr. Sweeney said. “And that makes me very angry.”

Mr. Sweeney, who was also seen as a reformer when he assumed the presidency of the AFL-CIO a decade ago, said he was particularly disappointed to see his former union, the Service Employees, leading the rebellion. “Pulling out of our convention dishonors the founders and the members of my union. It is a grievous insult to all the unions that helped us – and to the unions in this hall who came here to discuss and debate the difficult issues and make historic changes.”

The Teamsters and the Service Employees said they each pay about $10 million a year to the AFL-CIO. As the dispute over the federation escalated, both unions went into arrears. Mr. Hoffa said his union planned to devote about $5 million annually to a new labor coalition that could rival the AFLCIO, called Change to Win. The remainder, he said, will go to direct efforts by the Teamsters to organize new workers.

The dissident unions proposed that up to 50% of the per capita dues paid to the AFL-CIO be returned to the unions for organizing. The federation’s leaders and many unions resisted that idea, proposing a much smaller rebate.

“They’re basically living off the past, and I think we’ve got to change that,” Mr. Hoffa said. “We choose to ally ourselves with these unions that do not want to be bound by the past.”

The withdrawal will have a serious impact on the AFL-CIO’s bottom line, wiping out about a third of the national body’s annual dues. The Teamsters and the Service Employees said they want to remain affiliated with state labor councils and will continue to fund them. However, the federation has proposed kicking out unions that lack affiliation with the national organization.

“Let them cast the first stone,” Mr. Hoffa said yesterday.

Mr. Stern also faulted the AFL-CIO for an over-reliance on politics as the means to solving the problems facing workers.

“We need to spend money on growth and not rely on elected officials,” Mr. Stern said.

Comments like that have caused concerns for Democrats, who are the main beneficiaries of labor’s largesse and get-out-the-vote efforts.

However, Mr. Stern suggested that politicians would do better to be connected with fast-growing unions like his than the AFL-CIO, which he portraying as lumbering. “We actually spend more money on politics than what the AFL does,” Mr. Stern said.