OTTAWA -- Tom Mulcair is doubling down on his commitment to stay on as interim federal NDP leader despite mounting pressure for him to step aside ahead of a caucus retreat next week in Montreal.

In a statement on Thursday, Mulcair said he is committed to staying because the caucus voted last spring to keep him in place until a new leader is selected in the fall of 2017.

"Following our convention in the spring, our caucus asked me to stay on as leader until a new leader is chosen," he said. "After a thorough conversation with all caucus members, I committed to doing just that."

The leader's comments come as a number of party members -- including current and former members of the federal caucus -- whisper that it's time for Mulcair to quit as leader as the party attempts to rebuild in wake of its stinging 2015 election defeat.

Multiple sources, who spoke to The Canadian Press on the condition of anonymity for fear of being seen as openly critical of their leader, have confirmed a concerted push is underway.

"If he doesn't make a move on his own, he is going to face a revolt," said a former MP.

Veteran BC NDP MP Nathan Cullen, who is not running in the race to replace Mulcair, acknowledged internal matters like leadership need to be discussed as a team in Montreal.

"Let us get together, let us sit down and talk together and we will come together," Cullen said in an interview.

"I really believe it. It has been my experience with NDP caucuses and I think it will be my experience with this caucus."

The party also needs to ensure its own politics do not get in the way of its work, Cullen added.

"When you're talking about yourself in politics, usually you are losing ... Those internal conversations -- you can only be in them for so long. Canadians do not have the NDP caucus on their minds. Canadians have decent jobs and a clean environment on their mind."

In April, Mulcair faced NDP faithful in Edmonton and received a resounding message that he should step away as the permanent chief of the party -- a move that was unprecedented in Canadian political history.

The NDP caucus held a vote following the convention and all but two agreed Mulcair should stay on in the interim to lead the team of 44 MPs.

In an interview on Thursday, Quebec NDP MP Romeo Saganash said he continues to believe Mulcair should keep his job until a leadership convention is held late next year.

"I think he is the best that we have right now and he should remain," Saganash said.

Saganash said his caucus colleagues have a right to their own opinion on Mulcair.

"I have my own," he said.

Party sources say support for Mulcair dwindled this summer as many staff and caucus members examined the state of the party.

There's serious concern, for example, about party fundraising capacity and levels of public support for the outgoing leader.

Just this week, one public opinion poll pegged Mulcair's support at a mere 7.9 per cent and the leader has largely remained out of the public eye throughout the summer months.

There are also ongoing questions about the party's ability to raise money, especially as it tries to recover financially from the longest election campaign in modern Canadian history.

Elections Canada filings for the second quarter of 2016 indicate the NDP lost two-thirds of its donor base, which topped 48,000 in the run-up to last year's election when polls suggested the party was the front-runner.

In the second quarter of 2016, the NDP collected $1.08 million from 15,906 donors -- a far cry from the Conservative party which brought in $5.07 million from 37,223 donors. The Liberals, meanwhile, received $4.9 million from 36,080 contributors.

Another issue for the party is the NDP satellite office saga, which embroils Mulcair personally.

The party is currently using its own funds to fight a legal battle that centres on the use of satellite offices in Montreal, Toronto and Quebec City.

The NDP maintains that it never used the offices for non-parliamentary purposes and has been challenging a decision by the secretive Commons internal board of economy that ordered 68 MPs -- many of whom were defeated in the last federal election -- to pay back $2.7 million in funds that went toward the operations.

Mulcair faces hefty bill of more than $400,000 related to the satellite office controversy.