Tom Mulcair is not taking part in the election to choose his successor.

He has endorsed none of the four candidates and does not discuss the NDP leadership race, except to say he believes it has been more collegial, less peppered with personal attacks, than the one that he won in 2012.

"I am not even voting," Mr. Mulcair said during a recent interview in his Centre Block corner office, a room that overlooks the Ottawa River. It is important, he said, that he remains neutral until there is a new leader in place.

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But Mr. Mulcair also gives the sense, as he prepares to depart the job he has held for more than five years, that he feels underappreciated by his party. The rank and file handed him his walking papers in April of 2016, after the election the previous fall saw the NDP lose more than half of its 95 seats.

"People tend to forget that 44, which is the number of seats we got in 2015, is the second-highest number ever other than the Orange Wave" of 2011, Mr. Mulcair said.

And, he said, "no matter what else we can say about the great [NDP] leaders of the past – and the party has had nothing but great leaders – none of them was able to break through in Quebec" until he won the Montreal riding of Outremont in 2007 that had been Liberal since its creation in 1935.

Jack Layton was the party leader at the time, as he was during the improbable federal election six years ago that saw New Democrats take 59 of 75 seats in Quebec – a province that had all but shunned the party through five decades. But Mr. Mulcair was the architect of the NDP's Quebec surge.

His detractors, including some within his own caucus, accuse him of being autocratic, arrogant, even egotistical. His long stretch as lame duck of a party that does not appear poised to win an election any time soon has made it more difficult for the NDP to raise cash from donors. The next leader has a rebuilding job to do.

But Mr. Mulcair's time at the helm of the New Democrats and as a federal politician has not been without its successes.

Mr. Layton's death shortly after the 2011 election left Mr. Mulcair with the job of guiding a large caucus of young – in many cases, very young – neophyte MPs through the political minefield of their first year in office. There were remarkably few explosions.

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Through five years as leader, Mr. Mulcair has been widely praised, even by his political adversaries, for his "prosecutorial" performance in Question Period, and for the calm, cutting style he employed to hold both Conservative and Liberal prime ministers to account.

Above all in his career in federal politics, there was that heady election night of May 2, 2011, when the Montreal bar in which he planned to hold his victory party was deemed by party organizers to be too small to contain the thousands who would turn out to celebrate the NDP's wins, and when candidates he had enlisted across Quebec – some of whom were just names on ballots – were being told they had seats in the House of Commons.

That was the high point.

Four years later, at the end of a campaign he entered as front-runner, was the low. Four months after the 2015 election, Mr. Mulcair wrote a letter to NDP supporters taking responsibility for the election losses.

A postmortem conducted by party officials found that New Democrats blamed the central campaign for proposing "cautious" change as opposed to the "real" change they believed was offered by Justin Trudeau. The Liberals had said they would run deficits. Mr. Mulcair said he would not. New Democrats felt they had been out-lefted.

But Mr. Mulcair makes no apologies for campaigning to balance budgets.

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"I am never going to back down from my position that it's not responsible to future generations to buy votes with your grandchildren's money," he said.

"And, by the way, it is actually a position that the NDP adopted in Halifax in 2008, that our party will run on platforms of balanced budgets," Mr. Mulcair said. "It's not something I pulled out of thin air in 2015. Jack ran on balanced budgets. I ran on balanced budgets."

Nor does he regret defending a woman's right to wear a face-covering niqab during a citizenship ceremony – a position that saw the NDP drop 20 points in popular support in Quebec over 48 hours during the 2015 campaign.

"I am proud that I didn't betray my principles on that because I have fought for human rights my whole life," he said.

That position may have handed the election to Mr. Trudeau.

Being leader of the third party while Mr. Trudeau has been Prime Minister has been frustrating for Mr. Mulcair.

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"I found [former Conservative prime minister Stephen] Harper more respectful of me as a leader in Parliament than I find Mr. Trudeau," said Mr. Mulcair, who decried what he said are the large number of times Mr. Trudeau has skipped Question Period.

But the frustration will soon end. The results of the first ballot will be revealed on Oct. 1, and there will be a new leader by Oct. 15, at the latest.

"I am going to keep my seat for now," Mr. Mulcair said, "but I am in very advanced discussions with a number of universities and research institutions in Canada and I am keeping those options open."

His resignation as an MP would trigger a by-election in Outremont, the loss of which would be a demoralizing blow to the NDP.

"I was elected four times there," Mr. Mulcair said. "There is a good base. And, if there is a by-election, I sincerely hope we could hold onto it."

As for his own political future, Mr. Mulcair said he is "never running again at any level" of government.

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But "I hope to be the [former Ontario NDP leader] Stephen Lewis character who gets invited up to the stage [at NDP campaign events] to give a rousing speech in years to come because I so firmly believe that we should finally get a government that others have only talked about."