Olivia Chow’s long awaited jump in to the Toronto mayoral arena paves the way for a high-stakes byelection that should give Canadians their best preview to date of the 2015 federal battle between the Liberals and the NDP.

Now that Chow has stepped down as MP for Trinity-Spadina, Prime Minister Stephen Harper has until the fall to choose a date to fill the Toronto vacancy.

Chow’s riding is not Conservative-friendly. Over the past 25 years Harper’s party has never scored better than third place in Trinity-Spadina.

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But that does not mean that the prime minister’s calculations will be devoid of strategic considerations.

If Harper waits until the limit to call the byelection, the campaign would take place concurrently with the last stretch of the Toronto municipal election — timing that would be less than ideal from the perspective of Trinity-Spadina voters.

It may suit Harper to ensure that his opposition rivals have to compete with the mayoral battle for attention. But a late call stands to give the Liberals an organizational edge over the NDP.

With Chow running for mayor, the New Democrats may not be able to count on as many boots on the ground of Trinity-Spadina in the fall as they would this spring.

Helping Trudeau to secure more momentum for his party with a high-profile win against the NDP is certainly not on Harper’s 2014 to-do list.

On the other hand, a byelection held this spring could well coincide with an Ontario provincial campaign. From Harper’s partisan perspective, anything that takes some Liberal and NDP foot soldiers out of the provincial trenches is a bonus for Tim Hudak’s Tories.

Whenever the byelection is called, the stage will be set for a Liberal/NDP battle that may make last fall’s hard-fought duel for Bob Rae’s former seat of Toronto-Centre come across as a mere warm-up act.

Coming as it will a year before the general election — in a province where both the Liberals and the New Democrats are jostling to cast themselves as the alternative to the ruling Conservatives — Trinity-Spadina will be a major test for Trudeau and Mulcair.

In 2011, Chow won more than 50 per cent of the votes cast in the riding, but that kind of decisive number is the exception rather than the rule for Trinity-Spadina.

Since the late 1980s, the seat has swung from the NDP to the Liberals and back and most campaigns prior to 2011 were fought street by street.

If there is a constant it is that the Liberals tend to do well in Trinity-Spadina when they are doing well nationally. In 1993 — the year Jean Chrétien swept Ontario — the Liberals won the seat with more than 50 per cent of the vote.

If the NDP is to be seen as a serious contender for power in 2015, Mulcair can’t afford to lose any of his current Ontario territory between now and the general election. But it is just as important for Trudeau — who starts off from third place seat-wise — to continue to gain ground.

The Trinity-Spadina byelection will take place in the aftermath of the April 7th Quebec election.

If the PQ wins a majority next month the stakes could be even higher for the Quebec-based leaders of the NDP and the Liberals.

For Mulcair, it will be an occasion to road-test his contention that 50 per cent plus one is a high enough referendum bar to set the negotiation of Quebec’s terms of secession from Canada in motion. Justin Trudeau contends that a higher albeit yet unspecified threshold is in order.

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At the time of the debate on the clarity act in the late ’90s, New Democrat sympathies for the Liberal move to come up with federal rules for future Quebec referendums ran nowhere lowest — outside that province — than in Toronto’s NDP circles.

If Mulcair can’t sell his referendum approach in a downtown Toronto riding that his party has held in every election since Chrétien retired, he might be hard-pressed to sell it anywhere else outside Quebec in the 2015 election.

Chantal Hébert is a national affairs writer. Her column appears Tuesday, Thursday and Saturday.

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