Green Party Leader Elizabeth May doubled the size of her Green caucus this week. That still leaves her 10 MPs short of official party status and a long way from holding the balance of power in the House of Commons.

So what would it take to get her there — and what would that cost her rivals on either side of the aisle?

Paul Manly became only the second MP to be elected under the Green Party banner in Canada when he won the federal byelection in Nanaimo–Ladysmith on Monday. After he's sworn in, he'll be half of a two-person Green caucus — as large a Commons caucus as the party has ever seen.

But 12 seats is the threshold for obtaining official or recognized party status in Parliament, a designation that brings with it extra resources, committee spots and speaking time in the House. The Greens also probably will need to have more than just two seats if they're to hold the balance of power in a minority government — something May said she's hoping for following this October's federal election.

May is the country's longest-serving leader of a party with elected federal or provincial legislators. And perhaps for the first time in her tenure as Green Party leader, her goal of putting the Greens in a balance-of-power position is looking plausible. (It might look even more plausible if May is able to recruit former Liberals Jody Wilson-Raybould and Jane Philpott to her party.)

The Greens are now polling at just under 10 per cent nationally, a level of support the party hasn't consistently hit in the polls in over eight years.

In addition to their historic win in Nanaimo–Ladysmith this week, the Greens have seen provincial breakthroughs in elections in British Columbia, New Brunswick and Prince Edward Island, winning enough seats to hold the balance of power in minority legislatures. May can point to these examples in the fall when she asks Canadians for their votes.

But just how many of those votes would she need to achieve her goals?

Votes to the left and right, but mostly left

According to a breakdown of its latest poll provided by Abacus Data, any gains made by the Greens would come at the expense of parties across the spectrum. For every 100 votes that flip to the Greens, 36 would come from the Liberals, 28 from the Conservatives, 27 from the New Democrats and 10 from the Bloc Québécois and other parties.

The numbers suggest that the Liberals have the most to lose from any Green surge in the polls.

But the New Democrats would suffer disproportionately. Though the Greens might take more votes from the Liberals, and as many votes from the NDP as they would from the Conservatives, the New Democrats have fewer votes to lose.

Using the Canada Poll Tracker vote and seat projection model, it's possible to use these polling results to estimate what would happen to the current political landscape if the Greens continue to make gains.

Green gains come at NDP, Liberal expense

The model suggests the tipping point for the Greens is around 16 per cent support nationwide — just under seven points more support than the party currently has in the polls. That's where the seat projection model gives the party a greater than 50 per cent chance of winning at least 12 seats.

Those 10 extra seats — nine more than the party currently is projected to win — come primarily at the expense of the NDP. In this scenario, with the Greens at 16 per cent, the New Democrats drop eight seats, from 30 down to 22. The Bloc drops two seats, while the Liberals lose only two seats compared to the current projection, despite a steeper drop in vote share than the New Democrats experience.

The Conservatives, who fall by about two points nationwide in this scenario, actually see their seat total tick up by three, to 164. That still puts them short of the 170 seats required for a majority government, but slightly increases their chances of reaching that threshold.

Paul Manly won Monday's byelection in the B.C. riding of Nanaimo–Ladysmith. (Chad Hipolito/The Canadian Press)

Interestingly, the model suggests that 16 per cent for the Greens is also the tipping point for the Conservatives. As the Greens approach that mark, the Conservatives' odds of winning a majority improve. Above 16 per cent, however, the Greens start to eat into those odds as they make seat gains in Ontario and prevent Conservative inroads in Quebec and Atlantic Canada, where weaker Liberal, NDP and Bloc numbers initially benefit the Conservatives.

The point where the Greens' support allows the party to grasp the balance of power is harder to pin down. In the mid-teens in national support, the Greens might have enough seats to keep a Conservative-led minority government afloat — though it's hard to imagine the two parties agreeing on much when it comes to fighting climate change.

Before the Greens could hold the balance of power for a Liberal-led minority government, however, Prime Minister Justin Trudeau's party first would have to improve its current position in the polls. The Liberals currently are not in a position to win enough seats to assemble a working majority even with the support of NDP and Green MPs.

The Green path to a caucus dozen

The most likely path that would give the Greens 12 seats runs primarily through Liberal and NDP territory.

Based on this hypothetical Green surge, the Poll Tracker seat projection model suggests that the top targets for the Greens are all on Vancouver Island, where the NDP holds sway. In addition to Nanaimo–Ladysmith and May's riding of Saanich–Gulf Islands, the Greens have their best chances in the neighbouring seats of Victoria and Esquimalt–Saanich–Sooke.

The Green Party's next target likely would be the tier of seats largely held by the Liberals in places where the Greens' provincial cousins have been doing well lately. In addition to Cowichan–Malahat–Langford (another NDP seat on Vancouver Island), the next best targets are Guelph in Ontario, Fredericton in New Brunswick and Malpeque in Prince Edward Island. These ridings cover districts where provincial leaders Mike Schreiner (Ontario), David Coon (New Brunswick) and Peter Bevan-Baker (P.E.I.) won their seats.

Other Green Party prospects, according to the model, include Dufferin–Caledon in Ontario, Pierre Boucher–Les Patriotes–Verchères and Laurier–Sainte-Marie in Quebec and Halifax in Nova Scotia. But the results of provincial elections point instead to seats like Winnipeg Centre in Manitoba, Beauséjour in New Brunswick and Charlottetown in P.E.I. as giving the Greens an outside chance in this hypothetical exercise.

It's all speculation, of course. But as voters break new ground in elections from coast to coast, maybe now is the time for Greens to dream big.

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