DETROIT — Calibrating his approach for a tough head-to-head primary matchup in a year when many Democratic voters want to put up a united front, Senator Bernie Sanders of Vermont on Friday renewed his attacks on former Vice President Joseph R. Biden Jr.’s record on trade while also conceding that Mr. Biden could defeat President Trump.

In Michigan, where the Sanders campaign is running a TV ad featuring an autoworker who says his community has been “decimated” by free trade deals, Mr. Sanders attacked Mr. Biden for his support of the North American Free Trade Agreement in the 1990s.

“Joe Biden and I have a very serious disagreement with regards to NAFTA,” Mr. Sanders said at a round table he convened of workers, union leaders and economists in Detroit before a rally attended by thousands. “Just last year — I’m not talking about 20 years ago — Joe said that voting for NAFTA, quote, was not a mistake, end of quote, and a few years before that he called NAFTA a success,” Mr. Sanders said. “Joe, you’re wrong. NAFTA was not a success. Voting for it was a big, big mistake.”

As a long line of mostly young supporters filed into a downtown convention center, organizers called out for any union members to identify themselves. After major disappointments on Super Tuesday, Mr. Sanders is hoping for a comeback next Tuesday as six states go to the polls, none more crucial for him than Michigan, the first of several Midwestern industrial states to vote in the next two weeks. Four years ago, he pulled off one of the biggest surprises of his first presidential campaign when he upset Hillary Clinton in Michigan.