Two US trade unions representing more than three million workers have left the country's main labour federation, the AFL-CIO.
Dissidents criticise the leadership of AFL-CIO president John Sweeney
The Teamsters and the Service Employees unions say the AFL-CIO neglected the movement's loss of members and influence in favour of politics.
Another two unions are boycotting the current AFL-CIO convention in Chicago.
Correspondents say the US labour movement now faces one of the biggest splits in its history.
The breakaway unions have already set up a rival organisation.
Service Employees' International is the largest and fastest-growing union in the US, with more than 1.8 million members, while the Teamsters call themselves "the world's most powerful union".
The rebels are among seven dissident unions representing a third of the AFL-CIO's 13m members - and $35m (£20m) in dues.
Experts say a split in US labour could hurt the Democratic party, which generally gets strong union backing.
The split came at a convention meant to celebrate the 50th anniversary of the merger of the American Federation of Labor and the Congress of Industrial Organizations.
That merger ended an acrimonious split between the unions that had lasted since 1935.
Anna Burger said the AFL-CIO had not done enough to change
AFL-CIO president John Sweeney criticised the rebels at the start of the convention.
"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," he said to a standing ovation.
Long-simmering tension between the dissidents - part of a coalition called Change to Win - and AFL-CIO burst into the open on Sunday.
"Today, we have reached a point where our differences have become unresolvable," coalition chair Anna Burger said in a statement on the eve of the convention.
About 1,000 supporters of Mr Sweeney rallied on Sunday under the slogan United to Win, the AFL-CIO said.
The dissident unions are unhappy with Mr Sweeney's leadership. He is expected to win re-election without challenge at the convention.
The US labour movement has long been in decline.
According to government statistics, 12.5% of all US workers - and 8% of private-sector workers - are union members.
In the 1950s and 1960s, about a third of workers were unionised.