The federal NDP's rank and file turned on Tom Mulcair Sunday as deep ideological divisions beset the party in the wake of last fall's crushing electoral defeat.

After an unprecedented rejection of a political leader, the struggling third party must now decide what it stands for as it moves to replace him.

Mr. Mulcair received only 48 per cent support in a leadership review held in Alberta's capital city, a number that stunned to silence the 1,768 delegates assembled for the convention.

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"Don't let this very divided vote divide us," Mr. Mulcair appealed to his members after the loss.

The weekend was marked by increasing tension between climate-change activists and a vocal Alberta delegation that included NDP Premier Rachel Notley.

After fumbling an opportunity to distance himself from comments supporting a pipeline ban, Mr. Mulcair found himself in a furious debate over the Leap Manifesto, a proposal to wean Canada off fossil fuels and reject new energy projects. Alberta delegates resoundingly rejected the document.

The party has given itself up to two years to select a new leader, and Mr. Mulcair vowed to stay on until his successor is named.

"The disappointment from the election obviously is something that we're now going to leave behind us with a change at the helm," Mr. Mulcair said.

NDP MP Peter Julian said the mood was grim backstage as the results were announced.

"We were surprised and saddened, there's no doubt. There were tears," he said.

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In a speech before the vote, the NDP Leader tried to pitch himself as the scrappy fighter New Democrats need to remain united against the Liberal government.

"If you keep standing with me then together we will never stop fighting," Mr. Mulcair said during a half-hour speech where he vowed to protect universal health care and fight growing income inequality.

But his final, at times emotional, plea wasn't enough to hold on to his party's support. Many delegates who had said they were waiting to hear the speech before making up their minds said afterward that Mr. Mulcair did not deliver a new vision for his leadership.

Jerry Toews, a delegate from Edmonton, said he was undecided until the final moments.

"To be quite honest, I don't think he had as great a speech as [Notley]," he said of the Alberta Premier, who spoke at the convention on Saturday afternoon.

"I think that was a big part of the problem. What was his speech really saying today?"

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While the NDP has a long tradition of patience with leaders who lose, it wore thin with Mr. Mulcair, whose performance in last fall's federal election campaign left many members disappointed and angry. No federal party leader has failed to secure a majority when their leadership is in question.

Mr. Mulcair was brought in to lead the party in 2012, after the NDP won official opposition status under Jack Layton, who passed away shortly after the 2011 election.

The party hoped the NDP could build on Mr. Layton's success by selling Mr. Mulcair's moderate vision to Canadians. But the 2015 campaign proved a disaster after Mr. Mulcair ran on an uninspired platform that proposed balanced budgets and held off on tax hikes for the rich. Part of the party's current identity crisis stems from that campaign and the Liberal government's embrace of deficits and higher social spending. As a result, the NDP lost more than half its seats and fell back to third place.

Many attending the convention said they felt their views were ignored during the campaign, and didn't trust Mr. Mulcair to listen the second time around. "He ignored us," said John Hutton, a youth delegate from Halifax.

The leadership issue wasn't the only one at play. The Leap Manifesto, supported by nearly two dozen riding associations, dominated the discussion in hallways and on the convention floor.

In a divided vote on Sunday morning, delegates agreed to study the manifesto over the next two years in local riding associations before bringing it back to the 2018 convention.

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"Don't let this debate be the end. Let it be the beginning," filmmaker Avi Lewis, one of the document's originators, said before the vote.

But the manifesto was the cause of much consternation in the convention's home province.

With Alberta's NDP Premier enjoying an enthusiastic reception among delegates, the Edmonton convention was proof of how much has changed in the federal party since Ms. Notley's historic victory nearly a year ago.

Stephen Lewis, a former Ontario NDP leader and party elder, said he was speechless after Mr. Mulcair's loss. "I think the very large Alberta delegation here probably voted significantly against Tom Mulcair," he said. He cited Mr. Mulcair's comment that he was open to a pipeline ban as well as the Leap Manifesto's adoption for turning the Alberta delegation against the leader.

A day earlier Ms. Notley had called on the party to shun the manifesto and embrace pipelines, a pitch that was repeated by members of her party on the convention floor. "From the Premier, through cabinet and MLAs, we are all supporting what's in the best interest of Alberta," said MLA Jessica Littlewood after Mr. Mulcair's defeat.

Candidates who decide to enter the leadership race to replace Mr. Mulcair will first have to face the critiques that dogged him for the past six months: that his policies were too right wing for the NDP.

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Many in the NDP are now looking for a new direction, one that couldn't be provided by Mr. Mulcair, said Ontario MP Charlie Angus. "What's the vision for 2019?" he asked. "Tom just didn't deliver that."