At stake are millions of pounds of grants and donations funnelled from Unite’s accounts into political campaigning, as well as the union’s influence over the selection of future MPs and in policy votes.
One Labour MP said: “This is a massively important election to the future of the Labour Party and whether the wider movement is united against the Tories or divided amongst itself.”
The contest comes days after left-wing supporters of Angela Rayner backed away from launching a leadership challenge following the party’s narrow victory in the Batley and Spen by election.
Sir Keir’s allies now hope campaigner Gerard Coyne, who stood against Mr McCluskey in 2017, will clinch the post as the most influential trade union leader in the country.
Mr Coyne, the union’s former West Midlands regional secretary, has vowed to shift the union’s focus from being “a backseat driver for the Labour Party” to workplace issues affecting its 1.2 million members.
Against him are left wingers Steve Turner, who is endorsed by the Communist Party, and Sharon Graham, who has the support of the Socialist Workers Party.
Left wingers fear that a victory for Mr Coyne would mean less Unite money being showered on supporters of Jeremy Corbyn and fringe causes.
Mr Coyne has vowed to “throw open the shutters” on how the union spends members’ cash, including sums lavished on a £98 million hotel complex built in Birmingham whose costs ballooned.