By RON FOURNIERAP Political Writer

AFL-CIO president John Sweeney, 2nd left, secretary/treasurer Rich Trumka, executive vice president Linda Chavez-Thompson, and American Federation of State, County and Municipal Employees International president Gerald W. McEntee, hold hands at a solidarity rally, Sunday, in Chicago. Four major unions representing one-third of the AFL-CIO’s 13 million members decided to boycott the organizations convention, which begins Monday. (AP Photo/Brian Kersey) CHICAGO — Organized labor is at war with itself as the Teamsters and a major service employees’ union decide to bolt from the AFL-CIO, paving the way for two other groups to sever ties in the labor movement’s biggest rift since the 1930s.

The Teamsters and the Service Employees International Union, the largest AFL-CIO affiliate with 1.8 million members, intended to announce Monday that they are leaving the federation after failing to reform the 50-year-old labor giant, according to several labor officials who spoke on condition of anonymity.

The unions are part of the Change to Win Coalition, seven labor groups vowing to accomplish what the AFLCIO has failed to do: Reverse the decades-long decline in union membership.

But many union presidents, labor experts and Democratic Party leaders fear the split will weaken the movement politically and hurt unionized workers who need a united and powerful ally against business interests and global competition.

Two other Change to Win Coalition unions signaled their intentions to leave the AFL-CIO: United Food and Commercial Workers and UNITE HERE, a group of textile and hotel workers. But they were not scheduled to take part in Monday’s news conference, said the officials who declined to be named because they were not authorized to discuss the developments prior to the news conference.

The four dissident unions, representing nearly one-third of the AFL-CIO’s 13 million members, announced Sunday they were boycotting the federation’s convention which begins Monday, a step that was widely considered to be a precursor to leaving the federation.

‘‘Our differences are so fundamental and so principled that at this point I don’t think there is a chance there will be a change of course,’’ said UFCW President Joe Hansen. Leaders of the dissident unions say the AFL-CIO was beyond repair from within. In addition to seeking the ouster of AFL-CIO President John Sweeney, they demanded more money for organizing, power to force mergers of smaller unions and other changes they say are key to adapting to vast changes in society and the economy.

Sweeney, whose was expected win re-election this week, said he had met many of the dissidents’ reform demands, and suggested they had put their egos ahead of workers’ interests.

‘‘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,’’ Sweeney said. The rhetoric was unusually personal, in part because dissident leader Andy Stern of the SEIU is a former Sweeney protege.

Rank-and-file members of the 52 non-boycotting AFL-CIO affiliates expressed confusion and anger over the action. ‘‘If there was ever a time we workers need to stick together, it’s today,’’ said Olegario Bustamante, a steelworker from Cicero, Ill.