Bernie Sanders sat in a mostly empty room at a Detroit convention center four days before the Michigan primary on Tuesday, and he spoke softly about failure. “In the 1950s,” he said, “this was a wealthy, prosperous community. And then ….” His eight guests, people whose lives have been touched by shifts in American trade policy, nodded.

Then came the North American Free Trade Agreement, a broad treaty signed into law by Bill Clinton in 1993 that reduced tariffs and duties between the United States, Canada and Mexico. Mr. Sanders voted against it in the House — and against its recent replacement, the United States-Mexico-Canada Agreement, as a senator.

“I have done my best to oppose these disastrous trade agreements,” Mr. Sanders said later at a public round table with his guests — labor leaders, autoworkers, activists. “Joe Biden has supported them.” To this day, he added, Mr. Biden doesn’t consider his NAFTA vote a mistake.