Howard Beckett has quit the race to be Unite boss and backed rival left-winger Steve Turner for the top job.
The deal is aimed at thwarting the centrist candidate in the race Gerard Coyne, who has called the move a "stitch up".
Mr Turner and Mr Beckett, two of the union's assistant general secretaries, issued a joint statement saying they would now publish a "blended manifesto".
It means two left-wing candidates remain, including Sharon Graham, who has widespread support across the union.
Unite is one of the most powerful unions affiliated to the Labour Party and whoever emerges victorious in the contest on August 26 will become an influential figure in British politics.
Mr Turner and Mr Beckett's joint statement said: "Throughout this contest we have both been committed to one thing above all – developing further the role our union has played since its foundation as a fighting back, progressive, campaigning force for working people throughout Britain and Ireland.
“It is clear that developing that unique role requires the unity of the left in our union, and of all those representatives and members in the workplaces and beyond who have made Unite a union to be proud of.
“Howard Beckett has decided he will support Steve Turner as Unite’s next general secretary. Both recognise the vision and strengths of their respective campaigns and Steve Turner recognises the key manifesto commitments and energy generated by Howard's campaign.
"They will both work to implement a blended manifesto, taking the best ideas from both candidates, when Steve Turner becomes general secretary.
"Howard Beckett will campaign alongside Steve for the next two months to present a joint programme which includes greater support for workplace representatives, important new communications initiatives including Unite TV, upgraded education and training for members, an independent and progressive political voice, and a new structure for the union reflecting the diversity of our nations and regions."
Mr Beckett, who placed third in the contest, is seen by some as a divisive figure by some Labour MPs.
A BBC Newsnight investigation earlier this month revealed that he was involved in a bid to oust some sitting Labour MPs in 2018, including then deputy leader Tom Watson, and replace them with left-wingers.
Reacting to the news, Mr Coyne said: “I am the only candidate who would change the culture of the union, make its financial affairs transparent, improve its internal democracy and end the ill-judged attempts to drive the Labour Party from the back seat."
He added: “It’s now clear that if you vote Turner, you get Beckett.”
