With Stephen Harper already defining the Liberals and New Democrats as a coalition, the two opposition parties may have no choice — unappetizing as it might seem — but to form one.

The Conservative tactic is designed to discredit the Liberals. It builds on the antipathy felt in much of the country after the attempt in late 2008 to form a Liberal-NDP government with Bloc Québécois backing.

The fact that this attempt was both legal and constitutional mattered little. A big chunk of the electorate viewed it as unfair. Some voters in English Canada saw the decision to involve the separatist Bloc as treasonous.

When Michael Ignatieff replaced Stéphane Dion as Liberal leader, any possibility of a coalition appeared to collapse.

But now, as they prepare for a possible spring election, the Conservatives are using their considerable communications skills to resurrect the idea. From Harper to Finance Minister Jim Flaherty to the most obscure Conservative senator, “coalition” is a must word when referring to the opposition.

Conservatives now rarely speak of Liberals. In their parlance, Ignatieff is not Liberal leader. He is coalition leader — the man whose secret agenda is to form a government with socialists and traitors.

The difficulty the Liberals have is that, in one way or another, this is exactly what they would do should they win a plurality — but not a majority — of Commons seats.

They might try to rule as a minority government willing to make deals with one or more opposition parties — as Harper does now.

They might make a longer-term arrangement with the NDP, as Ontario’s Liberals did in 1985. Or they might form a formal coalition, as Britain’s Conservatives and Liberal Democrats did last year.

Any of these would be normal practice in Canada following a tight election. But the Conservative aim is to smoke the Liberals out before voting day.

Given the state of media, it is not inconceivable that this strategy might work — that the intentions of the opposition could become the defining issue of the next election campaign.

In which case, it might make more sense for the two national opposition parties to seize the initiative and — before the election — define the kind of arrangement they would like to see in a Liberal-led minority parliament.

Such an arrangement would not be an alliance of the left. There is nothing left-wing about the current Liberal party. In both foreign policy (the war in Afghanistan) and economic policy (the war on the deficit), the Liberals have more in common with the Conservatives than they do with the NDP.

Nor is the NDP particularly left-wing. Like the Liberals, New Democrats have supported much of the Conservative agenda, particularly on crime.

Rather, an NDP-Liberal coalition (for political reasons in English Canada, the Bloc could not be involved) would simply be an alliance of convenience, one aimed at ousting the Harper Conservatives.

Such a coalition could discomfit some right-leaning Liberal voters and send them toward Harper.

If history is any guide, a coalition would also damage any future electoral chances of the junior partner, in this case the NDP.

But given the mood of the country, an NDP-Liberal coalition properly arranged ahead of time (with, for instance, common candidates) might win the next election — which, for politicians, is what matters.

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With their anti-coalition propaganda, the Conservatives have already defined this coming contest in dualistic terms — good versus evil, us versus them.

If voters truly believe that they face a stark choice between Harper and the forces of Anti-Harper, the Conservatives could be unpleasantly surprised.

Thomas Walkom's column appears Wednesday and Saturday.

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