President Donald Trump said Sunday that Americans could feel “some pain” from the emerging trade war triggered by his tariffs against Canada, Mexico and China, and claimed that Canada would “cease to exist” without its trade surplus with the United States.

Canadian officials have said that the goods have been selected with an eye to maximizing the effect of their retaliation in Republican-controlled states, in a bid to get representatives from those states to weigh in with Mr. Trump and ask him to call off U.S. tariffs and de-escalate.

A day after levying steep new tariffs on Mexico, Canada and China, President Trump acknowledged what economists, members of Congress and even some of his own aides — in their previous lives — have been saying all along: Americans may find themselves paying the costs.