The joint leader of a new super-union is to back the MP Jon Cruddas for the job of the next deputy prime minister.
Mr Cruddas announced last year he will run for the job
Derek Simpson, general secretary of Amicus, told the BBC that other candidates for the job appeared to be "locked into the Blair legacy".
But he said the Dagenham MP was a "breath of fresh air" who he believed would follow the trade union agenda for major change in the Labour party.
Amicus and the Transport and General Workers Union are to merge in May.
Unions expect the Chancellor, Gordon Brown, to become the next prime minister and believe their best hope of influencing decision-making at the top is to have a strong deputy.
Mr Cruddas, 44, announced last September that he is to run for the job.
It is understood that other unions including the GMB are also supporting Mr Cruddas instead of the former union boss, now education secretary, Alan Johnson.
Mr Cruddas, a former Downing Street adviser, with roots in the trade union movement, is seen as the "outsider" candidate - the only runner who is not a minister.
Portrayed as a left-winger, he has described New Labour as tired, old and neglectful and says should he become deputy PM he would serve as a "transmission belt", reconnecting the party's grass roots with its leaders.
The merger of the two unions will create the country's biggest union with more than 2 million members - about one-third of all UK union members.