Local 832 of the United Food and Commercial Workers union — one of the biggest unions in the province with 16,000 members — came out in support of Selinger, in part because of his track record on labour issues.
"Under Greg's leadership, we have seen regular improvements in workplace health and safety, a dedicated minimum wage for security guards ... and annual increases to the minimum wage each and every year," said union president Jeff Traeger.
Up to 2,217 delegates will vote on the leadership of the governing New Democrats on March 8, and the UFCW is entitled to 160 of those slots. Selinger has already been endorsed by the Canadian Union of Public Employees, which has been allotted 288 delegate positions.
Selinger's competitors — former cabinet ministers Theresa Oswald and Steve Ashton — have secured support from smaller unions.
The labour vote is key for Selinger, because he has fallen behind his competitors in the fight for 1,300 delegate positions from NDP associations in the province's 57 constituencies. Ashton is by all accounts in the lead among constituencies, but the party has not released a tally and each campaign has offered up a slightly different count.
The hurdle for Selinger and the unions is finding people qualified to fill their allotted delegate slots under the party's rules. Each union local has to find a delegate who is both a member of the NDP and a member of that specific local. CUPE has more than 100 locals across the province and cannot fill a delegate slot in Brandon, for example, with a worker from Winnipeg.
Traeger said his union was still trying to get its 160 slots filled before the party's Wednesday deadline.
"We've actually (filled) over 100 and we're working very hard to get those numbers up by the deadline."
There are also no guarantees that union delegates will vote as their leadership has suggested.
"Certainly, we're not holding a gun to anybody, telling them how to vote, but we're giving them a strong recommendation and the reasons why," Traeger said.
The Oswald campaign is counting on getting support from some union delegates whose leaders are backing Selinger.
"The hundreds of individual union ... delegates are not required to officially declare who they are supporting before they vote for the next leader at convention," Jodee Mason wrote in an email.
Selinger was essentially forced into the leadership contest by a caucus revolt last October, when Oswald and four other senior cabinet ministers called on him to resign in order to help the party rebound in the polls. NDP support plummeted after the government raised the provincial sales tax in 2013, and an election is scheduled for April 2016.
Oswald, a former health minister, has said she is the party's best chance to win re-election. Ashton, a former infrastructure minister who did not take part in the revolt, has touted himself as the best person to reunite the divided party.
Selinger, who has seen his competitors win roughly two-thirds of the constituency delegates, said Monday he remains optimistic about his chances.
"We've got good support from many, many constituencies and lots of delegates. Like I said, we're right on track to what we thought was possible at this stage in the process."