The longtime ruling party of Canada’s energy-rich Alberta province lost its four-decade hold on power on Tuesday, ushering in a left-leaning government that has pledged to raise corporate taxes and increase oil and gas royalties.

The Alberta New Democratic Party swept enough districts to form a majority, taking most of the seats in both the business center of Calgary and the provincial capital of Edmonton, according to preliminary results from Elections Alberta. The outcome was a blow to Premier Jim Prentice’s Progressive Conservative party and one that threatens to roil the province’s economy amid a slump in energy prices.

“We need to start down the road to a diversified and resilient economy. We need finally to end the boom-and-bust roller coaster that we have been riding on for too long,” NDP leader Rachel Notley, who is expected to succeed Mr. Prentice as Alberta’s premier, said at a news conference.

It was the first general election in the province since 2012 and served as a referendum on Mr. Prentice’s eight-month tenure as premier. The chief issue was a budget plan to raise income taxes while holding the line on spending to compensate for revenue lost largely due to the pullback in the energy sector.

“As the leader of the party, I accept responsibility for tonight’s outcome. I also accept responsibility for the decisions that led up to this evening,” Mr. Prentice said at a news conference conceding the election. He said he would resign immediately as party leader and would also resign his seat in the legislature.

Although an election wasn’t due until next year, Mr. Prentice called the snap poll after announcing the controversial budget in March amid a drop in oil prices to six-year lows. It was harshly criticized by the NDP for sparing corporations from increased taxes and targeted by the right-of-center Wildrose Alliance Party for not cutting spending.

The NDP has long been a marginal force in Alberta’s traditionally conservative politics, but recent public opinion polls showed its popularity surging. In the campaign, Ms. Notley attacked Mr. Prentice for reinstating provincial health-care premiums and being too cozy with oil-patch interests.

In a move that spooked some energy company executives during the campaign, Ms. Notley raised the specter of increasing royalties levied on oil and gas production, although she said that her party would only consider that once crude-oil prices recovered from recent lows.

She also signaled her party wouldn’t support a proposed Enbridge Inc. crude-oil pipeline, called the Northern Gateway, which would connect Alberta’s oil sands with a planned Pacific coast terminal in British Columbia, telling a local newspaper that “Gateway is not the right decision.”

The ruling party also faced a fresh challenge from the right, which had threatened the PC dynasty in the 2012 elections.

Mr. Prentice, a former federal cabinet minister and investment banker, shocked Alberta’s political establishment in December by engineering a deal with the former head of the chief opposition party in which she and several Wildrose colleagues crossed the aisle to join the ruling PC party.

That move strengthened the premier’s hand in the provincial assembly but angered many on the right. The Wildrose party regrouped under a new leader, former federal lawmaker Brian Jean, and mounted a vocal challenge in the campaign for conservative voters unhappy with the centrist PCs.

The Progressive Conservatives party governed Alberta uninterrupted since 1971, making it the longest-serving governing party in Canadian history.
(Wall Street Journal)