Defying calls by the AFL-CIO for unity, five prominent labor leaders yesterday announced formation of an independent coalition to build union membership, the latest escalation of a fight increasingly likely to split the 13 million-member labor federation into two warring factions.

"We have seen the decline of organized labor through both Republican and Democratic administrations," said Teamsters President James P. Hoffa, one of the five leaders. "A political sea change in America will not take place until we rebuild our movement."

Republicans are watching the split within labor with relish, foreseeing a weakened political adversary. But labor experts are divided over the potential consequences of a fractured labor movement. Some suggested that internal competition could help to stop the steady decline in union membership, and all of those interviewed said defections from the AFL-CIO would not necessarily harm organized labor.

AFL-CIO President John J. Sweeney sharply criticized the five labor leaders' actions. "Now more than ever we need a united labor movement," he said. "The clearest path to growing the union movement and helping more workers form unions is by exercising our greatest strength: solidarity."

The five dissident unions -- the Service Employees International Union, the Teamsters, Unite Here, the Food and Commercial Workers and the Laborers -- have tried to persuade the central labor federation to shift resources into organizing and to strengthen the AFL-CIO's authority over its 58 member unions. Moreover, these unions want to defeat Sweeney, who is seeking reelection in July.

The dissident unions have repeatedly been thwarted because they represent only 35 percent of the AFL-CIO membership and have been unable to win over other supporters. Sweeney is virtually assured of reelection, and he has more than enough votes to defeat the dissidents' proposals when the AFL-CIO meets in Chicago at the end of July.

The dissidents' inability to win on any front has increased the chances that some or all will quit the labor federation. The leadership of the SEIU, Unite Here and the Food and Commercial Workers have begun the process to "disaffiliate," and the Teamsters are likely to do so at the next executive committee meeting.

The five union leaders said they have not yet agreed on staffing, a budget or officers for the "Change to Win Coalition," which will have the goal of organizing "the tens of millions of workers in the private sector who are desperate for a voice on the job." In a joint statement, the unions compared formation of the coalition to the creation in the 1930s of the Committees for Industrial Organization, which broke away from the American Federation of Labor to organize entire industries

Richard Freeman, a Harvard professor, and Elaine Bernard, director of Harvard's labor and work life program, separately said conflict within labor could be productive.

"Competition is very good for unions as well as for everyone else, and the last time unions grew was after the CIO split from AFL," Freeman said. "You can date the decline in union density from the time AFL and CIO became one [in 1955]. . . . The politics that led to this division is largely positive, but the new gang may not have much of a better strategy than the old gang."

Bernard pointed out that in Canada, the highest union density is in Quebec, where there are three competing labor federations.

Two other experts, Kent Wong, director of the Labor Center at the University of California at Los Angeles, and Richard Hurd, labor studies professor at Cornell University, said the 1930s, when the political environment favored organizing, is not comparable to today.

Wong said the SEIU brings to the coalition an understanding of how to organize in today's environment, noting that in Los Angeles and California, it recently organized 74,000 state home care workers and increased the percentage of janitors represented by unions from 20 to 80 percent, and of private-sector health care workers from 6 to 55 percent.