In 2015, Justin Trudeau’s Liberal Party — then the third party in the House of Commons — leapfrogged over Thomas Mulcair’s NDP to lead a coalition of progressive voters who had had enough of Stephen Harper’s Conservatives.

Trudeau was rewarded with a strong majority government as many progressive voters took him at his word, for example, when he stood in front of the TV cameras of the parliamentary press gallery and said: “We are committed to ensuring the 2015 election will be the last federal election using first-past-the-post.”

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By the time of election day in 2015, the NDP had already grimly acknowledged that the Liberals had done it again: They’d campaigned to the left to great success, scooping up the lion’s share of progressive voters desperate to prevent another victory by Harper and the Conservatives.

And then, as every NDP partisan at the time could have predicted, the Liberals proceed to govern from the right. Promises on electoral reform were ditched. As prime minister, Trudeau kept Harper’s greenhouse gas emission targets and even bought himself a pipeline. The SNC-Lavalin matter made clear that the control and grip of Trudeau’s PMO was every bit as tight as it was with Harper.

And so, with four months until another election day, many progressive voters who once embraced Trudeau are now pushing him away.

But this year, many disillusioned progressive voters are not looking to the NDP. They’re looking elsewhere.

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“The Green Party today, in my view, is a greater threat to the Liberals getting re-elected than the New Democrats are,” said David Coletto, CEO of polling firm Abacus Data.

The reason: A significant number of Canadians who voted Liberal in 2015 are telling Abacus and other pollsters that they are prepared to vote Green right now and a record number are prepared to consider voting Green.

Indeed, in a deep dive published late this spring that looked at the rise of Green support, Abacus measured the pool of accessible voters available to all parties. This is an important metric because it can give pollsters an indication of a party’s potential growth in a dynamic political environment and, conversely, it can signal that a party might have hit its ceiling and its vote intention numbers can only fall.

Three years ago, in August 2016, just 31 per cent of those surveyed were prepared to consider voting Green. By May of this year, 44 per cent have the Green Party on their menu of potential voting options. By contrast, the Liberal pool of accessible voters has dwindled from close to 70 per cent down to just under 50 per cent in the same period.

“This is a moment that is highly unique for the Green Party in public opinion in the vote intention world of us pollsters,” said Coletto, who characterizes voters who may be seriously considering two, three or even four different voting options this fall as “butterfly” voters. And these butterfly voters are a growing group.

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These are not just the findings of Abacus. The most recent poll from Ekos Politics, published June 17, has the Green Party in third place in the national horse-race number while the NDP has dropped to fourth. The Angus Reid Institute, in its most recent survey published June 10, also finds Greens moving up.

“The Liberal decline has benefitted the Green Party,” the institute said in its release. “This comes as more Canadians identify the environment and climate change as a top issue facing the country.”

The Greens, of course, recently doubled their presence in the House of Commons as a result of a byelection in the B.C. riding of Nanaimo—Ladysmith. Paul Manly joined party leader Elizabeth May, who represents B.C. riding of Saanich—Gulf Islands.

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That small but symbolically important win follows similarly important breakthroughs in recent provincial elections. The Greens are now the official opposition in Prince Edward Island. In New Brunswick, Greens hold three seats. And in Ontario last year, a Green candidate won that party’s first-ever seat at Queen’s Park. One other common denominator in all three of those elections was that Green support helped contribute to the loss by incumbent Liberal governments closely aligned in each case with the Trudeau Liberals.

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And, of course, in British Columbia, the Greens hold the balance of power with just three seats.

Coletto believes all of these elections, as well as the current federal polling climate, are an indication of an electorate ready to try new things, to think outside the same ballot boxes they’ve always ticked off.

“I think there’s an appetite for doing things we’ve never done before and a willingness to say — why not? And that willingness, if you’re the Conservatives or the Liberals or the NDP, is a scary proposition because once that starts to spread, you then get a wave. We saw that in 2011 with Jack Layton,” Coletto said. “Right now, there’s a mood in the country that none of the other mainstream parties are really authentic or real or doing it for the right reasons.”

May, who has been the Greens’ leader since 2006 and is about to fight her fourth general election as leader, believes that mood is precisely the factor that may make 2019 different from any other general election.

“What it feels like is that people want to vote for politicians and candidates that they feel are different, that are going to do politics differently, are going to keep our word to the voters (and) be of service to the country,” May said. “This is the election where people can really vote for what you want because we have a better chance than I’ve seen in a long time for a minority Parliament that results in co-operation and good government.”

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A Global News aggregation of polls by Abacus, Ekos, the Angus Reid Institute and Forum Research that were in the field in the first two weeks of June shows the Greens with 13 per cent nationally versus the NDP at 14 per cent. That same aggregation shows pockets of strong regional support that have the potential to grow into seat gains this October not just in B.C. but also, potentially, in Ontario, Quebec, New Brunswick and even Newfoundland and Labrador.

In B.C., for example, the Greens are the pick of about 21 per cent of voters, based on the Global News aggregation. But the Greens appear to have nearly that kind of strong support — 17 per cent — in Atlantic Canada.

Where the Greens poll lowest is the three prairie provinces, though May believes the Greens in Manitoba may surprise in the provincial election in that province set for Sept. 10.

And while even May herself thinks there is almost no chance of the Greens forming government in Ottawa this fall, she and many others believe there is a realistic chance they could move from their current holding of two seats in the 338-seat House of Commons to win official parliamentary party status at 12 seats. And with scenarios in which the NDP is reduced to 20-something seats and the BQ boosts itself to a seats number in the high teens, it then becomes quite possible that the Greens could be sought out to sustain a Liberal or Conservative minority government in a future confidence vote.

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A key question for the Greens as they campaign for more support — and other parties as they fight off any move to the Greens — is to figure out what motivates a Green voter. That process usually starts with a demographic analysis.

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So if we know that a Conservative voter is likely to be older and more likely to be male than voters of other parties and if we know a typical Liberal voter is likely to be more affluent and have higher education levels than voters of other parties, what demographic profile is emerging about this growing group of those who say they will vote Green or are considering voting Green?

Coletto said that it is hard to pin down a ’typical’ Green voter. While Green support does skew to a younger voter, Greens can be found in both rural and urban areas, in most regions of the country, and across other demographic cleavages.

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“What unites them is not demographics or socioeconomic status, it is values and what issues are important to them,” said Coletto.

Concern about the climate is one obvious value most Green supporters share. But Coletto said a surprisingly high number of those saying they would consider voting Green are doing so not because of the environment, but because of a promise to do politics in a different, less hyper-partisan way.

The federal Liberals, quite naturally, will be campaigning to renew their majority mandate and it seems clear they will have to prevent their support from leaking to the Greens in the same way — and perhaps in some more important ways — that the Conservatives must guard against leaking support to Maxime Bernier’s People’s Party.

The pitch by Liberals to those voters thinking about casting a ballot for the Greens or NDP will be fine-tuned in the coming months but, when asked last week what she would say to such a voter, Environment Minister Catherine McKenna underlined her party’s approach to governing an energy resource-rich country like Canada while trying to meet its international climate change targets.

“The pitch for the Liberal Party is that we understand the environment and the economy have to go together,” McKenna said. “You must appeal directly to people. You are trying to understand what this transition means including to people who may be worried about what it means to their future and for their jobs. And I think within our broader plan, you have to look at our climate plan.”

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In other words, the Greens are missing the balance the Liberals believe is their greatest virtue.

But there will be another pitch that Liberal parties have used and used often in general elections over the years: When a progressive voter votes for anyone other than the Liberal candidate, that progressive is helping to elect conservatives. And no progressive voter wants that to happen, right?

“Let’s be clear what the choice is,” McKenna said. “The choice is between our party that is working hard every day for Canadians and a Conservative Party led by Andrew Scheer that’s working really hard for conservative premiers.”

That argument has, at times, been a successful one but it is one that May and the Greens reject.

“I think voters now realize that if you want to see policies that deliver on climate, don’t waste your vote on any party but the Green Party,” May said last week in an interview from Victoria. “There’s only one party, only one leader, that’s been consistent. Our policies and our individual actions over decades make it very clear that we are honest and reliable. I wish I could say the same of my friends in the Liberal Party.”

But the Liberals will also have something going for them that the Greens, so far at least, have not been able to consistently demonstrate, and that is delivering at the ballot box what the pollsters say they should be delivering. To put it another way, the Greens tend to underperform on election day versus their numbers in polls taken in the days ahead of an election. Indeed, some pollsters are wary about measuring Green support precisely because of this chronic underperformance.

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Prince Edward Island was a good example, where any number of polls in the days before e-day had the Greens forming government but, in the event itself, they won official opposition.

Liberals, by and large, perform up to — and sometimes exceed — polling expectations largely because of a more professional, market-driven approach to voter identification, voter tracking and get-out-the-vote efforts. May, for the record, says she expects her party will also put more of an emphasis on the techniques of other professional political machines.

“But it won’t be based on cynicism,” May said. “That (demographic) slicing and dicing that you talk about. The idea that has been perfected by other parties that if there’s a wedge issue, you mobilize your vote and suppress someone else’s vote, we’ll never do that.”

And if the support pollsters find now for the Greens does tail off over the months ahead, the Liberals will disproportionately benefit.

Abacus asked those who said they were considering voting Green who they’d vote for it they did not, after all, vote Green and 34 per cent said Liberal, 27 per cent said NDP and 24 per cent said Conservative. And to test the appetite for strategic voting that these Green-considering voters have in a riding where only the Liberals or Conservatives had a chance to win, 58 per cent said they’d vote Liberal versus just 42 per cent for the Conservatives.

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David Akin is the Chief Political Correspondent for Global News.