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Canadian politics are not easily mapped on a simple left-right axis, of course: language and region always play their part. Nevertheless, it is striking that, despite having to split the vote with the CCF/NDP, the Liberals’ electoral performance, far from deteriorating, improved.

Photo by THE CANADIAN PRESS/Frank Gunn

Despite? Or because of? Perhaps what’s going on here isn’t simply two parties warring over a fixed slice of the vote. Perhaps, rather, the presence of two parties on the left (later three, and arguably four, with the advent of the Green Party and the Bloc Quebecois) has served to enlarge the total universe of voters available to them — a sort of political Say’s Law, wherein the supply creates its own demand.

By the conventional assumptions of politics, the splitting of the left-of-centre vote — at any rate, the non-Conservative vote — should have been fatal to the Liberals’ chances, delivering election after election to the Conservatives

The two parties, after all, while they have much in common, do not draw on the same undifferentiated mass of voters.

Though there is an overlap of Liberal-NDP “switchers,” each also has its own distinct base. Separately, then, the two command a larger total vote than they would combined, taking votes not only from each other but from the Conservatives.

The NDP, by its willingness to advocate for progressive issues the Liberals would prefer not to touch, has expanded the boundaries of permissible debate, pulling the median vote to the left, forcing the Liberals to respond and pulling the median vote to the left. At the same time, the broad philosophical sympathy between the two parties means they define the terms of debate, the default assumptions of public discussion, leaving the Tories permanently on the defensive, as the odd man out.