Canada’s rockbound political landscape has undergone a seismic shift with the election of a leftwing government in oil-rich Alberta, the country’s wealthiest and – until now – most conservative province.

The once-marginal New Democratic Party swept to victory in the western province on Tuesday night, humiliating the Progressive Conservative party that has ruled the province since the first term of US president Richard Nixon.

“We made a little bit of history tonight,” the province’s New Democrat leader, Rachel Notley, told supporters.

The result marks the latest and most surprising setback to prime minister Stephen Harper’s signature diplomatic effort to transport bitumen from Alberta’s tar sands to world markets through the controversial Keystone XL pipeline.

During the campaign, Notley promised to withdraw provincial support for the project, raise corporate taxes and also potentially to raise royalties on a regional oil industry already reeling from the collapse in world prices.

Notley led her party from a four-seat toehold in the provincial legislature to a commanding majority of 54 with a buoyant campaign that contrasted sharply with the flatfooted effort of the Progressive Conservatives under leader Jim Prentice, a former Harper cabinet member often touted as a future Conservative prime minister. Despite being one of a handful of PC candidates returned to office, Prentice resigned both his new seat and his leadership after the rout.

Canadian oil stocks slid slightly in response to the NDP win, with tar-sands giant Suncor Energy Inc losing 4.3% of its value in the first few hours of trading in Toronto before recovering half the loss by noon. The election of the NDP is “completely devastating”, declared financier Rafi Tahmazian of Canoe Financial LP in Calgary, Canada’s oil capital. “The perception from the market based on their comments is they’re extremely dangerous.”

But analysts say that the market itself that has done the most damage to the Canadian oil industry so far this year. Dozens of major projects in Alberta have been mothballed or delayed and thousands of workers laid off from high-cost tar-sands operations in response to plummeting oil prices.

With Barack Obama continuing to delay approval of the Keystone pipeline, which is designed to move Alberta bitumen to refineries in the southern US, Harper’s grand ambition to make Canada an “energy superpower” – despite widespread misgivings over the potential cost to the environment – was already foundering by the time of Notley’s ascension.

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Among other effects, Alberta’s unexpected revolution casts a shadow over the federal government’s imminent proclamation of sweeping new anti-terrorism legislation, which has provoked opposition from all corners of the political landscape.

Meanwhile, ordinary Canadians were reeling from the sheer magnitude of the shift in Alberta, which has placed the country’s most notoriously conservative province, taken for granted as an impregnable redneck kingdom, in the hands of its most progressive regional government. To explain the phenomenon, Toronto-based writer Doug Saunders asked his American Twitter followers to imagine socialist presidential candidate Bernie Saunders “becoming Texas governor by a big majority”.

Doug Saunders (@DougSaunders) Still, to put what just happened Canada in context for you Yanks, imagine Bernie Sanders becoming Texas governor by a big majority.

Most observers attribute the upset largely to local factors, including Notley’s inspired, informal leadership, a labour lawyer and daughter of the province’s pioneering social-democratic leader, Grant Notley. By contrast, the sitting premier had proven complacent and out of touch both during the campaign and in his half-hearted efforts to shore up a tired dynasty weakened by a succession of petty scandals.

At the same time, observers note significant limits on the new government’s hope to chart a course substantially different from its predecessor. Plunging prices have put the once-rich provincial government into deficit for the first time since the oil began to flow, and expectations of limitless prosperity remain high in a region that has recently enjoyed the country’s richest public services and its lowest taxes.