NDP Leader Tom Mulcair has lost the support of an Ontario labour group ahead of a critical leadership review next month.

The 30 members of Niagara Regional Labour Council passed a motion ‘to not support Tom Mulcair during his upcoming leadership review” next month in Edmonton.

“We feel, that under Thomas Mulcair, we've got the most right-wing leadership the NDP has had,” said Bruce Allan, vice president of the Council.

While the NRLC is not a large organization, the group expects other labour groups will join their calls for Mulcair to step aside.

“I'm certain there are other unions and union leaders who feel the same way,” Allan said.

Last week Sid Ryan, the former president of the Ontario Federation of Labour, said the New Democrats should find new leadership.

Mulcair has been fending off an internal revolt since his party’s brutal third-place finish in the last federal election.

Several riding executives, an Ontario MPP, a former NDP MP, and the party’s socialist wing have openly questioned Mulcair’s future.

Party sources tell CTV News that Mulcair has done little to regain support from the NDP’s traditional base of labour groups and activists.

And defeated NDP MPs say the bitterness of defeat still lingers, and Mulcair must step up and convince the party membership why he should keep his job.

“There's a wave of disappointment, so (Mulcair) must present himself as the person who can lead us going forward, and it’s really up to him to do that,” said Jack Harris, a former NDP from St. John’s, N.L.

High profile MPs Charlie Angus and Nikki Ashton have both avoided publicly endorsing Mulcair, deflecting to the upcoming vote by the membership.

Mulcair penned an open letter to the party shortly after the election and took responsibility for the NDP’s poor showing.

“I don't take anything for granted, I'm going to continue to work my tail off, but things are going well,” he said on CTV’s Power Play last month.

But NDP sources blame Mulcair for an underwhelming, centralized campaign along with his pledge to balance budgets instead of deficit spending like the Liberals -- a move Prime Minister Justin Trudeau gloated over Thursday at a Bloomberg forum in New York City.

“The night we had made that announcement and the left-wing New Democratic Party had announced they were going to balance the budget at all costs like the Conservative government, the day we said ‘no’ it’s time for us to invest in our country and (the NDP) confirmed they weren’t, I went home to my wife and said to my wife, ‘I’m pretty sure we just won the election,’” Trudeau said.

The NDP constitution says a leader can remain with support of 50 per cent plus one of the membership.

And while the party isn't known for turfing a leader after one election, there is grumbling among many of the grassroots that Mulcair took his party from frontrunner to third, and sweeping change should be considered.

Mulcair’s fate will be determined at the NDP convention the weekend of April 8 in Edmonton.