blamed avoidable blunders by Enbridge for the spill.

The Conservative opposition in Canada’s Parliament and the conservative government of Alberta have been pressing Mr. Trudeau to get Mr. Biden behind the pipeline. However, Annamie Paul, the leader of Canada’s Green Party, and several environmentalists say that Canada is backing the wrong side in the battle.

“We see no reason to doubt the data she is relying on,” Ms. Paul said of the governor. “It is ill-advised for our prime minister to be spending additional political capital down in the States on stopping the shutdown of a pipeline.”

Exactly how economically disruptive for Canada shutting the pipeline down would be is unclear.

Bob Larocque, the president and chief executive officer of the Canadian Fuels Association, a trade group for oil refiners, said that his members’ contingency planning has found other pipelines can handle about 60 percent of the oil that now arrives at Ontario and Quebec refineries through Line 5. The rest, he said, would have to be moved by truck, trains and ships, all more expensive transport modes. Mr. Larcoque said that he had no way to estimate the resulting increase in the price of gasoline and other fuels.

Under Michigan’s previous governor, Rick Snyder, a Republican, Enbridge received state permission to build a tunnel well under the lake bed. It would, according to the company, eliminate any danger to the pipeline from ships and also contain any oil in the case of any leaks.

Canada, Ms. Hillman the ambassador said, hopes that Enbridge can end the dispute by selling its tunnel plan to Ms. Whitmer.

“We’re really supportive of that tunnel project,” she said. “The pipeline’s already operated safely for over 65 years and that the tunnel project will make the pipeline safer, eliminating any risk of spills.”

Mary M. Chapman contributed reporting from Detroit.

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