Most Unite branches have backed Len McCluskey in his bid to be re-elected as general secretary, the union says.
The incumbent has been nominated by 1,185 branches - representing 559,000 members, more than five times the total of his nearest rival.
West Midlands chief Gerard Coyne is backed by 187 branches representing 98,000 members and Fujitsu rep Ian Allinson has 76 nominations covering 37,000 members.
Voting ends on 19 April.
Branch nominations will not have any direct bearing on the result, with each member voting individually for their chosen candidate.
Mr McCluskey said he had received an "incredible level of backing" and had opened up a "landslide" lead in his bid to be re-elected, while Mr Coyne said the nominations were "no guide to the eventual result".
The battle for control of Labour's biggest financial backer has at times been a bitter contest, and Mr McCluskey criticised the "cynical approach of one opponent", likely to be an attack on Mr Coyne.
The two candidates have traded blows throughout the campaign, with Mr Coyne accusing Mr McCluskey of focusing on "Westminster power games" rather than members' interests.
Unite has about 1,800 branches in total, and not all have chosen to make a nomination.
A date has not yet been set to announce the winner of the contest, which was brought forward by a year by Mr McCluskey's decision to resign before the end of his term office and stand again.