Canada’s steel industry has long shared Mr. Trump’s concern over steel from China and other countries in Asia coming into North America at what is viewed as unfairly low and subsidized prices. But the president’s plan to combat that with a tariff that might include Canadian exports has pitted workers on both sides of the border against each other. It has even divided families.

Cody Alexander leads the union local that represents workers at a former Algoma plant, now owned by Tenaris of Argentina, where about 650 people make high-strength pipes used for oil and gas drilling. In his downtown office overlooking the city’s relatively new hockey arena, Mr. Alexander said that his mother — now married to an American in Michigan — supported Mr. Trump’s tariffs. Mr. Alexander was wearing a red baseball cap with the slogan: “Let’s Make Algoma Tubes Great Again.”

“We kind of supported Trump’s trade talk during the election — and we made it very clear that that’s all we support him on — because we know how much foreign steel is affecting our markets.” Mr. Alexander said. “But the last thing I want to see is my guys get laid off again.”