The dairy dispute between the United States and Canada reached the highest political levels last week, as both President Donald Trump and Justin Trudeau, the Canadian prime minister, weighed in on the issue.

Trump, at an event in Wisconsin and later in Washington, sharply criticized Canada’s moves to reduce imports of ultrafiltered milk, which is used to make cheese.

"What they’ve done to our dairy farm workers is a disgrace. It’s a disgrace," Trump said. He called the...