It's cynical to assume that politicians who change their party affiliations always do so because they're placing self-interest over principle.

So it could just be a coincidence that Pierre Nantel — who announced Monday he'll be running as a Green candidate after winning two elections as a New Democrat — might have a better chance of winning a third time now that he's moved to a new party.

Like all New Democrat MPs in Quebec, Nantel's chances of re-election looked shaky heading into the October federal vote — his more than most, perhaps.

Nantel won his seat of Longueuil–Saint-Hubert, south of Montreal, by a margin of just 703 votes in 2015. He took 31.2 per cent of the vote as a New Democrat, barely edging out the Liberal candidate's 30 per cent and the 27.3 per cent won by the Bloc Québécois candidate.

That narrow 1.2-percentage point margin likely would be obliterated by the collapse in support for the NDP in Quebec under leader Jagmeet Singh. The CBC's Canada Poll Tracker puts the party at just 9.4 per cent in the province, a drop of 16 points since 2015, while both the Liberals and Bloc are polling in Quebec at about the same level of support they had four years ago.

This suggests both parties are in a good position to pick up a seat like Longueuil–Saint-Hubert from the NDP.

In other words, it would have taken a political miracle for Nantel, running as a New Democrat, to resist the shift in public opinion against the NDP in Quebec.

But could a move to the Greens really be Nantel's salvation?

Greens tied with NDP in Quebec

While it will be a struggle for the Greens to win any seats in Quebec — the party captured 2.3 per cent of the vote there in 2015 — Nantel could argue that he is jumping ship to a political vehicle with more momentum than the New Democrats enjoy.

The Poll Tracker puts Elizabeth May's Greens neck-and-neck with the NDP in Quebec, at 9.5 per cent support. The two parties have been jostling for fourth place in the province (behind the Liberals, Conservatives and Bloc) since the spring. Five of the last six published polls put the Greens ahead of, or tied with, the NDP in Quebec.

The trend line for the Greens has levelled off in recent weeks — but it's clear which party has been having a better year. The New Democrats started 2019 with 15 per cent support in Quebec, the Greens with just seven per cent.

Between April and June, the Greens raised about $33,000 from donors in Quebec who gave at least $200 each. The NDP raised only $20,000 from Quebec donors meeting that threshold. The Greens also have twice as many Quebec candidates listed on their website as the New Democrats.

Last month, NDP Leader Jagmeet Singh toured Quebec, where his party has experienced a significant drop in support since the 2015 federal election. (Jonathan Montpetit/CBC)

Still, the Greens have no base of support in Longueuil–Saint-Hubert, having captured just 2.5 per cent of the vote there in 2015. So while Nantel is very unlikely to win in Longueuil–Saint-Hubert as it stands right now, the Greens probably have a better shot at making further inroads in the province by Oct. 21 than the NDP does of turning around its long-standing negative trend line.

If the Greens do catch fire in Quebec, a credible candidate like Nantel might be one of the biggest beneficiaries.

And in the wake of breakthroughs by provincial Green parties in New Brunswick and Prince Edward Island over the last year, and the federal party's recent victory in the Nanaimo–Ladysmith byelection, the act of quitting one party and joining the Greens might actually be canny politics for some incumbents now. It hasn't been so in the past.

Nantel's floor-crossing predecessors unsuccessful

Nantel isn't the first sitting MP to cross the floor to the Greens (metaphorically, at least — Nantel will sit as an Independent in the unlikely event that the House of Commons meets again before the writ is dropped). Three have done it before. All three were subsequently defeated.

The first was Blair Wilson in 2008, a Liberal MP who was removed from caucus over allegations of election spending irregularities (he was subsequently cleared). When the Liberals wouldn't take him back, he crossed over to the Greens shortly before the 2008 election.

After capturing 37.5 per cent of the vote in 2006 under the Liberal banner in his West Vancouver–Sunshine Coast–Sea-to-Sky Country seat, he took just 14.4 per cent in 2008 as a Green. (He did double the party's previous score in the riding, however.)

Elected as a New Democrat in 2011, Ontario MP Bruce Hyer joined the Greens at the end of 2013. He was not re-elected in 2015. (Justin Tang/Canadian Press)

In 2013, Bruce Hyer joined the Greens after sitting as an Independent following his departure from the NDP caucus over his refusal to support the long-gun registry. He lost the race in Thunder Bay–Superior North in 2015, with his personal share of the vote going from 49.9 per cent to 13.8 per cent — though that was nearly five times the Greens' share of the vote in 2011.

Finally, José Nuñez-Melo stood as a Green candidate in the riding of Vimy in 2015 after not securing the NDP nomination in the seat he won in 2011. Nuñez-Melo's vote share fell by 43.3 per cent to just 2.4 per cent.

Clearly, Nantel will have to do much better than these three if he is to avoid their fate. But unlike them, he is joining a party that has managed a series of historic breakthroughs at both the provincial and federal levels in recent months. Perhaps electing a Green MP in Quebec will be the next one.

But it's a telling measure of the NDP's future in the province that one of the remaining members of its Class of 2011 has concluded he has a better shot at pulling off an unlikely upset with the Greens than being re-elected as a New Democrat in Quebec.