The United Fire Fighters of Winnipeg says Ashton is in the best position to unite the party following a caucus revolt in October.
Union president Alex Forrest says he respects the other candidates — Premier Greg Selinger and former cabinet minister Theresa Oswald — but says Ashton has a long track record of supporting labour.
The union has 25 delegates for the NDP leadership vote set for March 8.
Forrest says another two dozen firefighters are trying to get elected as constituency delegates in different areas of the province.
The only other union to weigh in so far is the Canadian Union of Public Employees, which is backing Selinger and could get up to 288 delegates.
As many as 2,217 delegates are expected to decide on the governing party's leadership at the March convention.