For those inclined to believe anything is possible in politics, Tom Mulcair or Justin Trudeau could be Canada’s next prime minister in 2015.

But no one should put any money on that, counsels retired University of Winnipeg academic John Ryan, an author and geographer who has recently been studying the near-term electoral prospects of the country’s centre-left parties.

Ryan, himself a lapsed New Democrat, has devised a scheme to challenge the inevitability of yet another Conservative win in two years time — in what would be Stephen Harper’s fourth successive victory.

An Ekos polling analysis last month warned: “The emergence of an NDP and Liberal Party, which are now operating at near parity, and a more popular Green party, constitutes ideal political arithmetic for the Conservative Party.”

Quite right, says Ryan, convinced that vote splitting on the left will again result in a minority share of votes, yielding another majority win for Harper.

Green Party leader Elizabeth May agrees, and stepped forward Saturday to declare that her party won’t run a candidate in a yet-to-be-called byelection in Labrador.

Former Conservative MP Peter Penashue stepped down in mid-March amid reports that Elections Canada is investigating his 2011 campaign spending.

May’s move could prove decisive given that Penashue beat his Liberal opponent by fewer votes than the Greens received.

In any event, May’s gesture will serve as a test run for opposition cooperation in 2015, despite the NDP’s refusal to play ball in Labrador.

Consider, says Ryan, that the NDP, although strong in Quebec, would have to score a serious breakthrough in Western Canada to win on its own steam in 2015.

For Liberals to win, Trudeau — likely to become the party’s new leader on April 14 — would have to surf to Sussex Drive on a wave of Trudeaumania big enough to woo Westerners.

Both scenarios are unlikely.

Still, with shiny new leaders, both opposition parties probably will do well in 2015, meaning they will productively divide the centre-left constituency.

Ryan’s plan to stave off that eventuality, of course, is not new.

Two B.C. MPs, in two separate leadership bids, have pushed for something similar — NDP leadership candidate Nathan Cullen in 2012, and Liberal leadership aspirant Joyce Murray in 2013. Both candidates claimed strong support.

Ryan’s proposal envisions both parties maintaining separate identities, but agreeing to a common platform on a few fundamental issues, plus an electoral strategy as follows:

• In ridings where the Conservatives lost in the last election, the sitting NDP, Liberal or Green MP would be the 2015 candidate.

• In seats that Conservatives or Bloc MPs now hold, the coalition would run the candidate from the opposition party that won the second-highest number of votes in 2011.

By Ryan’s analysis of Elections Canada data, New Democrats would win as many as 130 seats in 2015. Liberals, 70.

Conservatives would be reduced to 107 seats, and the Bloc would be eliminated.

It is worth noting that Ryan’s plan does not account for electoral upsets in any given riding, a not-infrequent event.

Nor does it consider the impact of 30 new Commons seats being added in suburban Alberta and Ontario, areas that lean Conservative.

But even if all 30 were won by Harper’s party, Conservatives — with 137 seats — still would have fewer seats than a coalition, claiming 200.

To date, Mulcair and Trudeau have done nothing but diss any notion of cooperation. But once Trudeau is firmly at the helm, the two leaders will be in a better position to act.

The big unknown? Voter reaction. If a groundswell of public support develops for a strategic cooperation, anything is possible in politics.

byaffe@vancouversun.com