LONGUEUIL, Que. -- NDP Leader Jagmeet Singh said his party is happy to welcome Quebec sovereigntists into its ranks, as the seemingly dormant independence question resurfaced on the campaign trail Monday.

Singh was questioned on the issue as he announced environmentalist Eric Ferland will be his party's candidate for the Quebec riding of Longueuil-Saint-Hubert, south of Montreal.

The announcement added a strange twist to the feud between federal New Democrats and Greens, who have been locked in a fierce battle for third place in public-opinion polls as each seeks to attract progressive voters unhappy with the governing Liberals and worried about climate change.

Not only is Ferland a former leader of Quebec's Green party, but the incumbent in the riding is former NDP MP Pierre Nantel, who defected to the Greens shortly before the election call.

In addition to switching parties, the two have something else in common: they're both sovereigntists who have advocated for Quebec independence, either currently or in the past.

While support for Quebec separation is polling low on voters' lists of priorities, a number of candidates have had to answer questions about their pasts as sovereigntists after deciding to run for federalist parties.

When asked about his past association with Quebec's sovereignty movement on Monday, Ferland said he's a federalist for the moment but could not rule out promoting Quebec independence in the future.

But he said for now, he'd rather focus on issues of climate change and social justice.

"The subject is sovereignty is not even on the table right now, it's far away, I see it's not moving forward so I prefer to work on things that move us forward," he said.

Singh, for his part, reiterated that he is a federalist but is happy to work with candidates from other political backgrounds as long as they share the party's progressive values.

"I believe in building a Canada that's united, that's my value," he said.

"I think it's important to recognize there is strong nationalism in Quebec that has a place in the New Democratic Party," he said, adding that federalists "couldn't ignore the desires of the people of a place."

Alexandre Boulerice, the NDP's deputy leader, also said he's one of the almost 50 per cent of Quebec voters who voted "Yes" in the 1995 referendum on separating from Canada. He said there's room for nationalists in the party.

In Longueuil-Saint-Hubert alone, at least three federalist party candidates have separatist pasts, including incumbent Nantel, who won his riding south of Montreal in for the NDP in 2011 and 2015.

The NDP dropped Nantel in August after it became known he was courting at least one other party, and he joined the Greens soon after.

Nantel's defection to the Green party led to uncomfortable moments for leader Elizabeth May, who was forced to answer questions about her new candidate's sovereigntist leanings last week after he told a radio host he wants Quebec to separate from Canada as soon as possible.

May initially said Nantel wasn't a separatist, describing him as "a strong Quebecer within the context of Canada," and said her party wouldn't endorse a candidate advocating for the breakup of the country. She later backtracked after he repeated the comments. Nantel remains a Green candidate and May has said she is comfortable with Nantel's position and how he fits in the party.

The Liberals are running former Parti Quebecois minister Rejean Hebert. The former provincial health minister has renounced sovereigntism, saying Quebecers don't want another referendum.

On Monday, Singh described Ferland's decision to turn New Democrat as evidence that his party's environmental plan "is the most complete," even as he sought to underscore what he described as the main differences between the NDP and the Greens.

"We believe in the importance of tackling the climate crisis. We know how important that is and we are going to do similar things," he said.

"But where we differ is we're not going to leave workers behind. Where we differ is we have a solid position on a woman's right to choose. With New Democrats, there's no question what you're getting. You know you're going to get candidates that all unequivocally support a woman's right to choose and will defend that and will also expand access to abortion services."

Ferland described his decision to run for the NDP as "obvious" given the party's plan for tackling climate change, which includes ending fossil-fuel subsidies, providing more incentives for the development of clean energy and expanding the price on carbon to include Canada's largest emitters.