Federal NDP leadership hopefuls lined up Sunday to take shots at rival Jagmeet Singh during the eighth and final debate in the campaign to replace outgoing Leader Tom Mulcair.

Singh, an Ontario legislator, is the only one of the four candidates who is not a federal politician, and he bore the brunt of the attacks in what was otherwise a relatively congenial faceoff between rivals whose campaigns overlap more than they diverge.

Manitoba MP Niki Ashton, who appeared by live feed because of flying restrictions related to her pregnancy, questioned Singh's dedication to the federal NDP because of his unwillingness to commit to running for a seat in Parliament if he loses the leadership race.

"I will be winning, and once I win, I will run federally," Singh said, prompting some cheers from the audience. "I'm absolutely running federally. No question about it."

Ashton's responded with a reminder: "I want to acknowledge that it's up to the members to decide who is going to win this race."

The campaign's two other candidates, Ontario MP Charlie Angus and Quebec MP Guy Caron, were also present for the debate.

Angus criticized what he described as Singh's plan to restrict old age security, the federal government's largest seniors benefit program.

"I still can't get over the fact that you would put a means test on old-age security at $70,000," Angus said.

"Clearly you can't hear what I'm saying. I'm saying I'm not changing — I'm enhancing the program," Singh replied.

"I don't think you agree with (old age security). I think you agree with letting seniors living in poverty," he told Angus, prompting gasps from the crowd. "Because if you don't understand the plan, how can you criticize it?"

124,000 eligible to vote

The NDP announced late last month that party membership had more than tripled since March, from 41,000 to 124,000 card-carrying New Democrats.

Singh's team says it signed up 47,000 new members and Elections Canada data show his campaign raised more than $350,000 since he entered the race in May, dwarfing his rivals' second-quarter fundraising numbers.

The considerable common ground that exists between the four candidates was apparent during the debate as they lambasted the Liberal government on everything from renegotiating the North America Free Trade Agreement to dealing with rising economic inequality and addressing climate change.

Diversity issues also came up as Ashton commended Singh, who is Sikh, for his handling of a racist heckler who interrupted a meet-and-greet last week.

A viral video of the incident shows a woman accusing Singh of promoting Shariah law and supporting the Muslim Brotherhood, as Singh tells her he supports her and will not be intimidated by hate.

"It's something that happens to racialized people across not just Canada, across the world," Singh told reporters after the debate when asked about the experience.

"There are often times that people are faced with unfairness and injustice, and they respond with all sorts of grace and with dignity."

Singh added that he hopes the incident shows people he is able to respond well in unpleasant scenarios.

"There's a principle that my mom taught me about. It's called 'chardi kala.' It's the idea of maintaining optimism in the face of adversity," he said. "I hope I maintained 'chardi kala' in that moment."

The first round of voting for the new NDP leader is scheduled to begin Sept. 18, with first-round results to be announced Oct. 1.

Candidates will have a final chance to pitch members for support next Sunday during a caucus retreat in Hamilton, Ont.