Justin Trudeau and Thomas Mulcair will probably have to work together after the next election, whether they like it or not.

The NDP leader mused this week about the possibility of co-operating with other parties in order to remove Stephen Harper from the Prime Minister's Office, after having ruled out a formal or informal coalition with Liberals in the past.

Trudeau, meanwhile, has dismissed the idea. But the electoral math suggests they will have little choice but to co-operate after the 2015 federal election.

If the results of that vote are similar to what polls are showing to be Canadians' voting intentions now, the Conservatives will be defeated. The Liberals hold a lead over the Tories worth roughly seven points, with New Democrats well behind in third.

But the NDP is still retaining a considerable level of support, particularly in comparison to where the party has traditionally stood. This prevents Liberals, despite their lead in the polls, from being in a position to win a majority government.

Assuming current trends hold, Liberals would instead likely win a plurality of seats and have the opportunity to form a minority government. Unless there is a serious collapse in support for either the Tories or NDP, a majority is virtually out of the question. That means the Liberals under Trudeau will need the support of another party to pass legislation.

It is extremely unlikely Trudeau would be able to (or be interested in) turning to Conservatives for votes in the House of Commons. That leaves Mulcair's NDP as the only plausible dancing partner. Whether they come together formally or not, Trudeau and Mulcair will have to co-operate to govern the country in such a scenario.

Of course, there is plenty of scope for change between now and the next election. Polls almost two years out from the next vote are hardly a reliable forecast (though that does not necessarily mean the results will be different).

Nevertheless, it is difficult to imagine a scenario in which Tories, by then in power for almost a decade, would be able to win another majority government against two strong opposition parties.

If the Conservatives manage to instead win a plurality of seats but not a majority, it is again likely Trudeau and Mulcair will have to work together. After the petty partisanship we have witnessed in Ottawa over the last few years, neither Liberals nor the NDP will be keen to give Tories their support in the House of Commons after 2015.

But unlike 2008, the moral legitimacy of a Liberal-NDP coalition toppling a Conservative minority would be easier to defend. The proposed arrangement between Stéphane Dion and Jack Layton at the time was, in effect, a "coalition of losers." Even Michael Ignatieff called it that in his memoirs of his time as Grit leader. The Liberals had lost seats and vote share, the NDP made only marginal gains, while the Conservatives received an expanded mandate.

A Conservative minority victory in 2015, however, would be entirely different. It would mean the Tories would have lost seats and votes. The Liberals would likely have gained a good deal of seats and support, while the NDP would still probably put up the second-best performance in its long history.

In this context, Harper's Conservatives would appear to be the "losers," while the NDP and Liberals would easily have taken over 50 per cent of the vote. The Bloc Québécois will no longer be needed for an awkward ménage-à-trois.

Unless either Trudeau or Mulcair wants to be the one to keep Harper in power, they will have to work together. For Harper, it is majority or bust. And they all probably realize this.

But politics is all about pretence. Discussing the potential for a future coalition, whether formal or informal, is an admission that neither Trudeau nor Mulcair might be able to win a majority government on their own. It distracts from discussing a party's strengths and policies. It gives ammunition to the government. Avoiding the topic is just smart politics.

But in 2015, after the ballots are cast, reality will set in.

Éric Grenier taps The Pulse of federal and regional politics for Huffington Post Canada readers every week. Grenier is the author of ThreeHundredEight.com, covering Canadian politics, polls and electoral projections. You can pre-order his eBook, "Tapping into the Pulse", a retrospective of polling in 2013, here.

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