Liberal Leader Justin Trudeau said Wednesday that he was "unequivocally opposed to any sort of coalition" with the NDP. That makes perfect sense. It would be far more logical for the Liberals to make common cause with the Conservatives.

Speaking to reporters in Halifax, Mr. Trudeau sought to clean up a messy quote that he had left behind the day before, in which he suggested he might be willing to co-operate with the NDP, provided Thomas Mulcair was not the leader. This seemed churlish.

Wednesday, the Liberal Leader declared once and for all that he was categorically opposed to a coalition with the NDP, in part because "there's too many big issues on which the NDP and the Liberal Party of Canada have deep disagreements."

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He's right. Under Mr. Trudeau's leadership, the Liberals on most major files have become virtually indistinguishable from Stephen Harper's Conservatives.

On the question of taxes, for example, the Liberals would retain all the Conservative measures, save for a minor income-splitting tax cut. The NDP, on the other hand, would raise corporate taxes.

On the environment, Mr. Trudeau appears content to allow the provinces to lead the fight against global warming, as does Mr. Harper. Mr. Mulcair is committed to compulsory national standards to reduce carbon emissions.

On natural resources, Mr. Trudeau backs the proposed Keystone XL pipeline and supports oil sands development, while Mr. Mulcair opposes Keystone and talks of a "Dutch disease" of oil dependency.

On national security, the Liberals support Bill C-51, the Conservative anti-terrorism legislation that the NDP opposes. The Liberals are also behind the Canadian military training mission in Ukraine, which the NDP insists must first be approved by Parliament.

Neither the Liberals nor the Conservatives support the proposed $8-billion NDP child care program. The Conservatives prefer direct payments to parents. The Liberals are silent on the issue.

And on Quebec separation, both the Conservatives and the Liberals back the restrictive Clarity Act, while the NDP endorses the Sherbrooke Declaration, which would make it much easier for Quebec to separate.

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Ideologically, then, it would make far more sense for a minority Conservative government to seek the support of the Liberals on a case-by-case basis, than for the NDP and Liberals to seek common cause.

Why, then, does Mr. Mulcair repeatedly offer to form a coalition with the Liberals? Because it profits the NDP leader to remind the 60 per cent of Canadians who want to see the back of Mr. Harper that the NDP is prepared to do whatever it takes to oust the Tories, but Mr. Trudeau won't go along.

In the next election campaign, Mr. Harper will warn that anything short of a Conservative majority government will lead to an Liberal/NDP coalition. Mr. Mulcair will enthusiastically agree. Mr. Trudeau will seek to change the subject.

The actual outcome of a hung parliament, if there is one, will be governed by the First Law of Coalitions, which states that coalitions are legitimate if they are legitimate and illegitimate if they are illegitimate.

If, on election day, either the Liberals or the NDP are several points behind the Conservatives in the popular vote and well short of them in the seat count, a coalition will be impossible, however much constitutional theorists might huff and puff.

As Michael Ignatieff put it, any coalition between two parties who did badly in an election would be a "coalition of losers."

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If, on the other hand, one of the parties is only a point or two shy of the Conservatives in the popular vote and a few seats in the count, then some type of NDP/Liberal co-operation would be inevitable, whatever Mr. Trudeau might say. The grass roots of both parties would demand it.

In the meantime, any day in which Mr. Mulcair gets traction arguing for a coalition to oust the Conservatives is a good day for the NDP. Any day in which Mr. Trudeau is forced to go on the record opposing coalitions is a bad day for the Liberals.

Both days are good days for Stephen Harper.