A battle over the future of the labor movement that will play out this week pits a 54-year-old iconoclast willing to break rules and tradition against two labor chieftains in their seventies who are deeply committed to union solidarity.

With the AFL-CIO in a decades-long decline in membership and prestige, and dissidents threatening to bolt, the stakes in the power struggle at a convention in Chicago are enormous. Federation President John J. Sweeney is expected to win election to a fourth term. But he and his chief ally, Gerald W. McEntee of the American Federation of State, County and Municipal Employees, face the threat of major defections from a coalition of insurgent unions led by Andrew L. Stern, president of the Service Employees International Union.

Stern, a charismatic and combative leader with roots in the civil rights and antiwar movements of the 1960s and 1970s, is considered likely to pull his union out of the AFL-CIO, and may take three other major unions with him.

The United Farm Workers union said Friday it intends to join Stern's Change to Win coalition, although it intends to remain part of the AFL-CIO.

Stern is challenging his one-time mentor, Sweeney, who preceded him as president of the SEIU. In addition, Stern's SEIU has challenged organizing in Iowa and California by McEntee's AFSCME, setting the stage for a bare-knuckles battle between the nation's No. 1 union, with 1.35 million members, and the No. 2 union, with 1.31 million.

Stern and his allies want to convert the AFL-CIO from a loose federation of 56 unions into a far more powerful institution that would be authorized to reward unions that bring in new members and punish those that do not, grant exclusive rights to those unions that dominate representation in a given employment sector, and effectively force mergers to build far fewer but stronger unions.

The insurgents would shrink the federation's Washington headquarters and shift millions of dollars into organizing drives.

The unions considering joining Stern in a walkout are the Teamsters, the Food and Commercial Workers, and Unite Here, which represents hotel and needletrades workers. Terence M. O'Sullivan, president of the Laborers International Union, is part of the Change to Win coalition but, like the farm workers, will not leave the AFL-CIO.

Four of the dissident unions are threatening to boycott the Chicago convention. If they do, they will withhold nearly $7 million in back dues. If they disaffiliate, they take about $35 million from the AFL-CIO, which has already been forced to lay off more than a quarter of its 400-person workforce.

The conflict has been building to a crisis for three years. Sweeney, 71, won the AFL-CIO presidency in 1995 on a promise to halt labor's decline, but he has been unable to do so. The share of the nation's workforce represented by unions has dwindled from 35 percent in 1955, when the American Federation of Labor (AFL) and the Congress of Industrial Organizations (CIO) merged, to 12.5 percent, in a pattern similar to that in most industrialized countries. During Sweeney's tenure, the share of the workforce in unions has fallen from 14.9 to 12.5 percent, and in the private sector from 10.3 to 7.9 percent.

This decline is compounded by unions fighting each other to organize the same workers. Sweeney's consensus approach to leadership, according to Stern and other critics, stops him from stepping between competing unions. The AFL-CIO is seeing unions suffering severe membership losses -- such as the steelworkers and autoworkers -- trying to organize food processing and health care employees in drives that splinter organized labor, critics say, instead of strengthening it.

Sweeney describes Stern's insurgency as "a move for whatever reason to gain power."

"They want to be leading the labor movement," he said. "What they don't understand is that a federation is not like a national union. A federation is just that -- it requires consensus among the affiliates in order to reach common ground."

Sweeney and McEntee began their careers in the 1950s, when the movement was still strong and labor ranked high among institutions respected by the public. Both come from a tradition in which steadfastness is a prized commodity.

"Those were the days we had some loyalty," McEntee said, pointing to a photograph on his office wall of a huge, multi-union picket line outside a Las Vegas casino with many of labor's current adversaries marching together.

Stern's threat to leave the AFL-CIO, in McEntee's view, breaks a cardinal rule. "When it became 'my way or the highway,' I think Andy lost not only support that he could have gotten, but his audience," McEntee said.

Stern began working in the union movement in 1973, a turning point for labor in America. The threat of global competition in autos and steel became abruptly apparent in the early 1970s, when business surprised labor with a suddenly tough, adversarial posture, starting in 1972 with the formation of the Business Roundtable.

"The people in charge of the AFL-CIO came up when running a labor union was managing success," Stern said. "For us," he said, referring to himself and other, relatively younger union presidents, leadership "has meant managing decline."

"They see it as all these different tactical ideas, 'let's spend a little more on organizing,' 'let's spend a little more on politics,' " Stern said about the AFL-CIO leadership. "It's like an orchestra where everybody is playing a different song and there is no conductor who says in the end, 'This is the concerto we want to play.' " Stern has led an increasingly aggressive assault on the labor establishment, violating a host of union rules and traditions:

* Stern and the SEIU have openly defied AFL-CIO rulings granting other unions, especially AFSCME, exclusive collective bargaining rights.

* In California, the SEIU dismissed objections of the AARP and other members of the labor-liberal community and lobbied against a nursing home residents' bill of rights to gain bargaining rights in the industry.

* Stern has accused union leaders of backing failed protectionist policies. "To try to stop the wage drops, unions have been an anti-competitive force, protectionist," Stern told the magazine Human Resources Outsourcing in an article that has angered many manufacturing union executives.

* In the last election cycle, the SEIU topped the list of donors to the Republican Governors Association at $575,000, raising eyebrows throughout the decidedly Democratic labor movement.

At the same time, the SEIU has been one of the most successful unions in building new membership. It has been on the forefront of groundbreaking strategies to organize building service and security workers, and converting home health care workers from low-wage consultants into unionized state and county employees with significantly improved pay and benefits.

The strategic and intellectual underpinning of the Stern insurgency can be found in a 9,500-word analysis written in 2002 by Steven Lerner, SEIU's director of building services organizing, called "Three Steps To Reorganizing And Rebuilding The Labor Movement."

The paper laid the groundwork for many of the insurgent proposals, calling for a strengthened labor organization empowered to accelerate union mergers to create large unions with exclusive bargaining powers in industry sectors and a prohibition on unions going out of their core sectors in organizing workers.

Key leaders in manufacturing unions view the Stern-Lerner analysis as a recipe for disaster.

"Our losses have been occurring in our core industries [metals and glass], where, with a hint of a plant shutdown, you are done," said steelworkers organizing director Mike Yoffee. The insurgents are seeking "exclusive rights within the industries which are much easier to organize" and to bar the steelworkers from these fields "will kill us."

Unite Here's John Wilhelm dismisses such concerns, noting that the 87 percent of workers not in unions offer "ample opportunity. . . . The task we have in this country today is to make those jobs that by their nature can't be exported -- hotel jobs, health jobs, janitorial jobs, retail stores, the enormous industry of distribution centers, truckers and warehouse workers -- into good jobs that support families."