CHICAGO, July 24 - Leaders of four of the country's largest labor unions announced on Sunday that they would boycott this week's A.F.L.-C.I.O. convention, and officials from two of those unions, the service employees and the Teamsters, said the action was a prelude to their full withdrawal from the federation on Monday.
The schism is the biggest rift in labor since the 1930's, when the Congress of Industrial Organizations, which was trying to unionize mass production workers in automobiles, steel and other industries, split off from the American Federation of Labor, which largely represented elite craft workers. This week's labor convention here was supposed to be a celebratory occasion marking the 50th anniversary of the merger.
Instead, the gathering has been marked by a split, the culmination of a rancorous debate within the union movement that also threatens to hurt labor's efforts in lobbying and in political campaigns, a development that concerns Democrats.
Leaders of the service employees union, the food and commercial workers union, the Teamsters and Unite Here, which represents apparel, hotel and restaurant employees, said they were shunning the convention because, in their view, the federation under the leadership of its president, John J. Sweeney, has been ineffective in halting the decades-long slide of organized labor.
Deepening the rift, the leaders of the four dissident unions, as well as the leaders of the laborers and the United Farm Workers, said that as a protest they were giving up their seats on the federation's executive council and would not seek re-election at the convention this week.
Two of the unions said they would take a further step.
Officials from the Service Employees International Union and the International Brotherhood of Teamsters who insisted on anonymity because a formal announcement had not been made said their unions would announce on Monday, the day the convention begins, that they were quitting the federation. The service employees, with 1.8 million members, and the Teamsters, with 1.4 million, are two of the biggest unions in the A.F.L.-C.I.O. They contribute $20 million each year, or about one-sixth of its budget.
In addition, Joe Hansen, the president of the United Food and Commercial Workers, indicated that his union would probably also leave, despite Mr. Sweeney's efforts to persuade them to stay.
"We are certainly willing to listen to anything they want to say," Mr. Hansen said. "Our differences are so fundamental and so principled that I don't think there will be any change in course."
A rift could hurt the labor movement badly by redirecting its focus and energies to internal battles instead of bedrock issues like fighting for wage increases and extending health care to more workers. Democrats, a traditional ally of organized labor, are especially worried that a schism might hurt their party's chances by making labor a less potent political force.
Andrew L. Stern, president of the service employees, defended the boycott, saying drastic changes were needed to save labor from further decline; the percentage of private-sector workers in unions has sunk to less than 8 percent from 35 percent a half-century ago.
"We're not trying to divide the labor movement - we're trying to rebuild it," said Mr. Stern, who is known as one of labor's most charismatic figures, though he has at times been accused of arrogance. "We have to do everything in our power to help workers. But when you're going down a road and it's headed in the wrong direction, and you know where the road ends, you got to get off the road and walk in a new direction where there is hope."
In unusually vituperative language, several union presidents accused the insurgents of betraying the labor movement, asserting that their boycott was sabotaging the cardinal notion of labor solidarity.
"Today is a tragic day because those who left the house of labor are weakening our house, and shame on them," said Leo Gerard, president of the United Steelworkers of America. "Solidarity is more than saying, 'My way or the highway, and if you don't do it my way, I'll take my marbles and run.' "
The four unions boycotting the convention represent about one-third of the A.F.L.-C.I.O.'s 13 million members. The A.F.L.-C.I.O. is the nation's main labor federation, a grouping of 56 unions that coordinates union activities in politics and often serves as the voice of American workers on job safety, raising the minimum wage and other issues.
The dissident leaders asserted that the federation, as structured, was not equipped to halt labor's decline. They said they were looking to a group they formed, the Change to Win Coalition, to encourage union growth with, for instance, multiunion organizing drives against large companies, including Federal Express and Wal-Mart.
Mr. Sweeney voiced dismay, bordering on anger, with the boycotters' actions.
"Not to attend the convention, especially when the differences that remain between our proposals are so narrow, is an insult to their union brothers and sisters, and to all working people," he said. "It's far easier to tear down a union movement than to build one. America's working people cannot afford for unions to declare 'it's my way or the highway' when workers are under the biggest assault in 80 years."
Gerald McEntee, president of the American Federation of State, County and Municipal Employees, said many union leaders had tried to persuade the unions not to walk out.
"I think everybody is hurt by this," he said. "This hurts American workers. This hurts the trade union federation that is the center of the planet for workers around the world."
Mr. McEntee, who is chairman of the federation's political committee, said the walkout would weaken labor's political clout by weakening its role of coordinating nationwide campaign activities for unions. In the 2004 election, polls showed that voters from union households accounted for 24 percent of all votes and that those households gave Senator John Kerry, the Democratic presidential candidate, a 5.8 million-vote majority.
"I think the only one who wins from this is George Bush and his minions who are trying to weaken labor unions," Mr. McEntee said.
The dissident leaders argued that they were boycotting over matters of principle, saying that under Mr. Sweeney the federation was shrinking from the drastic actions needed to make labor grow again. Those leaders have demanded that Mr. Sweeney agree to refund half the federation's budget to individual unions so they can spend that money to organize more workers.
Mr. Sweeney asserted that he had substantially increased the federation's spending on union organizing, but that rebating half its budget would cripple activities in politics, job safety and other areas.
Several union leaders said the dissidents were engaged not in a fight over principle, but in an ego trip and power grab.
"If you want attention, if you're looking to turn the spotlight on yourself, this is the strategy you follow," said Harold Schaitberger, president of the International Association of Fire Fighters.
Many of the convention delegates planned to celebrate Mr. Sweeney's re-election to a new four-year term, which is a foregone conclusion. Until this week's developments, several dissident leaders had signaled they would not consider leaving if Mr. Sweeney, 71, stepped down. He has headed the federation since 1995.
Ron Gettelfinger, the president of the United Auto Workers, had at first flirted with the dissidents, sharing some of their criticism that the federation was not doing enough to turn things around. But on Sunday he criticized the boycott.
"The house of labor is always stronger when it is united," Mr. Gettelfinger said. "It won't be the end of the world, what they're doing. It will just make the challenges we face that much harder."
Sunday's preconvention events often seemed like competing pep rallies. At an event to support Mr. Sweeney's re-election, a band blared, "Solidarity Forever," and hundreds of delegates wore T-shirts saying, "the Sweeney Solidarity Team - One Strong Voice for Workers Rights."
Before 2,000 Sweeney supporters, Linda Chavez-Thompson, Mr. Sweeney's running mate for executive vice president, laid into several entities that she said had sought to weaken labor - the Bush administration, the United States Chamber of Commerce, Wal-Mart - and then she surprised her audience by adding, "the Change to Win Coalition."
Mr. Sweeney criticized the convention boycott, saying, "It's a shame for working people that before the first vote has been cast, four unions have decided that if they can't win, they won't show up for the game."
The Change to Win Coalition held a news conference, with hundreds of union members attending, that served in ways as a kickoff meeting for the new group.
"The exciting news is that the Change to Win Coalition is about one thing: new hope for millions of workers in economic crisis," said Anna Burger, the coalition's chairwoman. "All of organized labor sees this crisis. Our coalition, however, believes that we can do something about it, and has a bold plan of action to make real change."