Green Party Leader Elizabeth May and Nantel made the announcement together in his Longueuil Que., riding on Monday.
The announcement came just three days after the NDP ejected Nantel from its caucus on Friday citing “confirmed reports” that he was in talks with another party to join their ranks.
Nantel had been a prominent and outspoken member of the NDP’s Quebec wing since winning his seat in the Orange Wave in 2011.
He was already confirmed as the party’s candidate for Longueuil-Saint-Hubert when ejected.
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There had been reports over the past year that Nantel was looking to join another party, with some suggesting he could look to the Bloc Québécois.
But on Friday, Quebec radio personality and former provincial legislator Bernard Drainville tweeted that he had been told by sources close to Nantel that the NDP candidate was in discussions to possibly join the Greens but that no decision had been made yet.
Nantel would be the third MP for the Greens, joining May and Paul Manly, who won the Nanaimo-Ladysmith byelection earlier this year in what marked a stunning defeat for the NDP as the party continues to struggle in the polls under Leader Jagmeet Singh.
Polls suggest the party is having particular difficulty in Quebec, with many of the MPs who won during the Orange Wave facing diminishing prospects of holding onto their seats in the fall campaign.
Roughly one-third of the party’s incumbents are not running again in 2019.
The party is also struggling to draw in fundraising dollars, recently falling behind the Greens.
Second-quarter fundraising numbers show the Greens raised $1.44 million from 14,600 donors during that time period.
In comparison, the NDP raised $1.43 million from 14,900 donors.