Not that long ago, British Columbia and Alberta enjoyed a pretty good relationship.

During Gordon Campbell's decade-long reign as premier from 2001-2011, B.C. and Alberta signed the ground-breaking Trade, Investment and Labour Mobility Agreement. The inherent value in their new economic alliance was notable and noted. Later, the two provinces became founding members of the broader New West Partnership Trade Agreement, which would eventually include all the Western provinces.

Those days seem so long ago now. Today, relations between Alberta and B.C. are about as bad as they've ever been. And they are possibly about to get even worse.

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Gary Mason: B.C. political makeover could leave the province unrecognizable

It all began to fall apart under B.C. Liberal Premier Christy Clark, who had a famously frosty relationship with former Alberta premier Alison Redford. When Rachel Notley was elected Premier of Alberta, Ms. Clark used the occasion to play partisan politics, citing the alarming level of debt Alberta was accumulating under the NDP. The B.C. Premier even went as far as using her government's Speech to the Throne to take petty shots at Alberta, a fact that was duly noted on the other side of the Rockies.

At one time it was thought that an NDP election victory in B.C., were it to ever happen, would usher in a new era of respect and civility between the two provinces, especially if the NDP in Alberta were in power at the same time. It doesn't look like it's going to work out that way.

Far from being ideologically simpatico, the nascent government of NDP Leader John Horgan, propped up by the Greens, could lead to the electoral demise of the NDP in Alberta. Talk about irony of ironies.

In the recent B.C. election, Mr. Horgan ran on a promise to use "every tool in the tool box" to kill the Kinder Morgan Trans Mountain pipeline expansion. But even as he said it out on the campaign trail (which he rarely did), many felt it would not be his hill to die on. He would go through the motions of opposing the project, it was thought, but never position himself in front of a bulldozer to ensure ground wasn't broken.

But then suddenly, he needed the help of Andrew Weaver and the Green Party to topple the Clark government (which is yet to happen, but could soon). Mr. Weaver and his followers are far more committed to ensuring the pipeline never gets built. And the Green leader doesn't care how crass, pompous and inflammatory he comes across in defending his viewpoint.

This week, Mr. Weaver took issue with Ms. Notley's position that there are many jobs at stake in B.C. if Trans Mountain doesn't go ahead. He said those jobs were a "myth" and added that British Columbians were "sick and tired of being told the 20th-century economy is the economy of tomorrow."

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But he really set people in Alberta off with his declaration that there was a lot that could be done to stop "the shipping of diluted bitumen in our coastal waters." The word our stood out, like the coastal waters off the West Coast didn't belong to all Canadians. Mr. Weaver is going to soon understand that everything he says will be examined (and torqued and twisted for partisan gain) like never before.

Needless to say, this has all become fodder for Ms. Notley's political enemies. Wildrose Leader Brian Jean said B.C. is now captive to "eco-radicals." One of his senior colleagues, Derek Fildebrandt tweeted that the "inmates are running the asylum" in B.C. He also publicly agreed with the suggestion that if B.C. stops the pipeline "it's political war."

What Mr. Weaver and Mr. Horgan have done is hand Ms. Notley's political enemies a big stick with which to club her. Those same opponents are now saying that the broad range of environmental measures her government brought in to obtain "social license" for pipelines was a sham and an unnecessary financial and regulatory burden on Albertans at a time when they could least afford it.

Mr. Weaver, the greenhorn, could soon learn that actions have consequences in politics. By killing Trans Mountain, or at least attempting to, the Green leader could give rise to the Conservatives in Alberta, who will be touting an environmental agenda drastically at odds with Ms. Notley's NDP. The national climate deal could also become a victim of all this. And we would unquestionably see the further deterioration of an important interprovincial relationship that had been strong for years.

And Mr. Horgan and Mr. Weaver will largely have to wear that.