The NDP finally has a leadership race, and now the candidates must decide how they'll handle a divisive issue inside their party: pipelines.

The only New Democrat in power anywhere in Canada, Alberta Premier Rachel Notley, backs the construction of two major pipelines, Trans Mountain and Keystone XL, to help her province's ailing oil industry. But the first candidate in the federal NDP leadership race, B.C. MP Peter Julian, is campaigning against new pipelines.

In May, B.C.'s provincial election will be a ballot-box test for New Democrats on pipelines: Provincial NDP Leader John Horgan has staked out a position against the Trans Mountain pipeline in his campaign against B.C. Premier Christy Clark, and the outcome could affect the course of the federal NDP leadership race.

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Not all the leadership candidates are making pipelines a central campaign theme. Northern Ontario MP Charlie Angus launched his campaign on Sunday focusing on economic inclusion, arguing that Prime Minister Justin Trudeau betrayed promises to make life better for the middle class. He also talked about protecting the environment, but his main theme was the concerns of ordinary folks trying to get by.

But oil pipelines will be an unavoidable issue for NDP leadership candidates in 2017.

The competing forces within the party on the issue made themselves felt at the party's convention in Edmonton last April, pushing leader Thomas Mulcair from one side to the other, then out of the leadership. Mr. Mulcair, who was fighting to retain his leadership in a review vote, sought to curry the support of leftist activists who penned the Leap Manifesto, which called for rejecting new oil projects and pipelines – then he found Ms. Notley's Alberta NDP delegates turning out in numbers to turf him.

This time, the federal NDP has decided not to hold any of their leadership debates in Alberta, the only orange province in Confederation. That way, perhaps, they can keep clashes over pipelines down to a dull roar, and Ms. Notley can avoid the spectacle of her own party attacking her policies.

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Many in the federal NDP think there's an opportunity in taking an unequivocal stand against oil pipelines. After all, Justin Trudeau won a lot of left-leaning, environmentally conscious voters in 2015 with a promise to balance the environment and the economy; many were upset when he approved the twinning of Kinder Morgan's pipeline from Alberta to Burnaby, B.C. Some New Democrats think it's time to take a firm anti-pipeline stand. Mr. Julian is vying to lead them.

Of course, there's still the problem that Ms. Notley, the leading figure in the NDP right now, shares Mr. Trudeau's policy: Both argue that pipelines can be balanced with a carbon tax and other emission-reduction policies so that climate change can be tackled while the energy sector is developed.

And the big problem is that the NDP still has to figure out if it can have an unequivocal anti-pipeline policy and still retain the now-dented dream of taking power in Ottawa. The B.C. election is likely to shape opinions.

Mr. Horgan, after all, is running in a province with a long NDP history and many environmentally-conscious voters. And he's running against a Liberal government that has been in power for 16 years. If his pipeline opposition hurts him, it will be a message to the federal NDP.

And it's Ms. Clark's Liberals who are likely to try to make a campaign issues out of the pipeline. It's not really a question of approving Trans Mountain – that's a federal matter, and Mr. Trudeau has given the green light – but Ms. Clark's party wants to portray the NDP as a risky bunch incapable of managing the economy, and they will use Mr. Horgan's opposition to the pipeline to argue he would scare off investment and jobs.

The B.C. NDP always struggles to find a balance between its environmentally-conscious "greens" and its "browns" – voters, often unionized private-sector workers, who are more interested in the economy and pocketbook issues. Now the federal NDP's leadership candidates will be forced to find a balance on a touchstone issue that divides their party.