Hostilities broke into the open on Sunday when four large unions announced that they were boycotting the annual convention of the AFL-CIO, which got under way in Chicago yesterday. It then emerged that two of the four were likely also to withdraw from the alliance altogether.
The two departing unions, the International Brotherhood of Teamsters and the Service Employees International Union, have 1.4 and 1.8 million members respectively. Their revolt represents a body blow - both in morale terms and financially - to the AFL-CIO and was being described as the worst rift in the labour movement since 1930.
It risks further undermining union strength in the United States which has been in decline for decades, eroded by dwindling membership rolls, the effects of automation, the relentless rush to improve productivity, and the ramifications of globalisation and increased world competition.
The AFL, the American Federation of Labour, and the CIO, the Congress of Industrial Organisations, broke away from one another in the 1930s. However, they merged again in 1955 and this year's convention was meant to be a celebration of that 50-year marriage. In all that time, the AFL-CIO, roughly equivalent to Britain's TUC, has been the voice of organised labour in the country.
But while roughly one in three workers in the private sector belonged to a union in America half a century ago, only 8 per cent do so today. The relentlessness of that decline and the political environment now makes union power still less credible and has given rise to the tensions of today.
A splintering of the labour movement also bodes ill for the Democratic Party, which for generations has depended on labour leaders to galvanise voters as well as raise funds for candidates for the White House and Congress. Last year, almost a quarter of all the votes cast in the presidential race came from union households, a majority of which supported the Democrat, John Kerry.
Part of the battle is being fought over John Sweeney, leader of the AFL-CIO for 10 years, who is expected to win re-election this week. Activists have been fighting to have him removed, arguing he has failed to re-energise the movement.
Seven unions now belong to an ad hoc grouping - the "Change to Win Coalition" - opposed to Mr Sweeney's re-election. All may end up breaking ranks. They are demanding new leadership and more funds to allow individual unions to merge and launch new membership drives.
"The AFL-CIO, to its credit, has listened to us. But in the end, they have not heard us. The language of reform has been adopted, but not the substance ... We have reached a point where our differences have become unresolvable," said Anna Burger, the chairman of Change to Win.
Backers of Mr Sweeney, 71, have come close to branding their departing brethren as traitors to the cause. "Today is a tragic day because those that left the house of labour ... are weakening our house and shame on them," growled Leo Gerard, president of the United Steelworkers, which will remain in the fold.
Others contended that the dissidents, far from helping, were playing into the hands of those who would like to see the movement grow weaker still. "I think the only one who wins from this is George Bush and his minions who are trying to weaken labour unions," said Gerald McEntee, president of the American Federation of State, County and Municipal Employees.