Three branches of Britain's biggest rail union are attempting to switch their allegiance from Labour to Tommy Sheridan's Scottish Socialist Party.
Tommy Sheridan was the Scottish Socialist Party's first MSP
The executive committee of the Rail, Maritime and Transport (RMT) union will be asked to endorse the move next week.
The branches are in Glasgow, Edinburgh and in Motherwell and Wishaw - First Minister Jack McConnell's constituency.
However, a Labour source warned the union to think "very, very carefully" before taking such action.
Earlier this year RMT general secretary Bob Crow said members were getting "fed up" with New Labour.
He suggested in June that the union could switch its support to the SSP.
The RMT's annual conference relaxed its rules during the summer to allow local branches to support other parties.
It also agreed to cut the union's annual
funding to Labour to £12,500.
In its original guise as the Amalgamated Society of Railway Servants, the RMT helped found what is now the Labour Party.
RMT's Scottish organiser Phil McGarry said: "Our union is proud of the role it played in helping found the Labour Representation Committee in 1899.
"But many of the arguments being made by the Scottish Socialists are clearly in line with our union's position and are finding favour with branch members in Scotland.
"They are arguing for a return to public ownership of the railways, the repeal of anti-union laws and are opposing the Scottish Executive's promotion of PPP/PFI
Mr McGarry expects the union's national executive committee to endorse the move by the three Scottish branches.
He said other branches were also considering their position, although he thought some would continue their Labour affiliation.
Mr Sheridan said he welcomed the move by the three branches.
"We would appeal to all trade unionists who believe in public ownership, the removal of nuclear weapons and the redistribution of wealth to back the only party in Scotland that promotes these principles - the SSP."
However, a Labour Party source warned: "I think the RMT should think very, very carefully before they take action.
"They certainly have an important and valued role in the direction of Labour Party policy.
"Outside the party they would have no influence whatsoever."
The SSP increased its representation in the Scottish Parliament from one seat to six at the May election.