When Donald Trump threatened to “break” the North American Free Trade Agreement, auto industry workers offered up some of the loudest cheers.

Mr. Trump easily won the Republican primary in Michigan this month. The state, home base for the American auto industry, also delivered an upset victory to Bernie Sanders, the Democratic anti-Nafta standard-bearer.

But the autoworkers’ animosity is aiming at the wrong target. There are still more than 800,000 jobs in the American auto sector. And there is a good case to be made that without Nafta, there might not be much left of Detroit at all.

“Without the ability to move lower-wage jobs to Mexico we would have lost the whole industry,” said Gordon Hanson of the University of California, San Diego, who has been studying the impact of Nafta on industries and workers since its inception more than two decades ago.