The party's appeals committee has rejected a complaint from Theresa Oswald that the Canadian Union of Public Employees has been given 288 delegates to next month's leadership contest.
The party is not commenting on the decision and has not provided reasons for the ruling to the media.
CUPE leaders have endorsed Selinger in the race, and CUPE delegates will account for 13 per cent of all votes on March 8.
The Oswald campaign had said CUPE was only entitled to one delegate under the party's constitution, because its locals across the province are not formally recognized as affiliates of the provincial NDP.
Oswald, Selinger and Steve Ashton are by all accounts in a tight race.
Meetings to choose delegates in each of the province's constituencies are being held over the next three weeks.