The man vying for Len McCluskey's job at the helm of Britain's biggest trade union said the leader should not be Jeremy Corbyn's "puppet master".
Gerard Coyne criticised the Unite general secretary, who is seeking re-election, for meddling in Westminster politics.
Mr Coyne, currently Unite's West Midlands regional secretary, told the Observer: "I just don't think that ever again the general secretary should be the puppet master of the leader of the Labour Party.
"There is an opportunity for change, for a fresh start, for members to get their union back."
Mr McCluskey's five-year term is due to end in 2018 but he opted to bring the election forward by a year to spring.
He told the Press Association he wanted to continue "coming after bad bosses" and fight for decent pay and conditions.
He added that Unite's support for Mr Corbyn was the decision of the union's executive and members, not a personal decision.
Meanwhile, the Sunday Telegraph reported that Theresa May is resisting calls from Tory MPs to curb trade unions' right to strike.
The calls come as a train drivers strike, in an argument with Southern Railway over the introduction of driver-only trains, caused vast disruption across London and the South East.
British Airways cabin crew, Virgin Atlantic pilots and Post Office workers are also planning industrial action in a Christmas of discontent.
It has also emerged that Sean Hoyle, president of the RMT union, which is co-leading the train strikes, said unions are acting together to "bring down this bloody working-class-hating Tory Government".
His comment, first reported by the Sunday Times, was made at a National Shop Stewards Network meeting in Brighton in September.