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 Joe Biden’s record on trade while also conceding that Biden could defeat President Donald 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, Sanders attacked 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,” 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,” 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 centre, organisers called out for any union members to identify themselves. After major disappointments on Super Tuesday, 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.

“We are in the midst of a very, very difficult primary process,” Sanders told reporters before taking the stage at his rally. “Come Tuesday, maybe Michigan is the most important state.”

Super Tuesday: Lively scenes as results flood in across 14 states Show all 29 1 /29 Super Tuesday: Lively scenes as results flood in across 14 states Super Tuesday: Lively scenes as results flood in across 14 states An anti-dairy protester is led away after storming the stage at Joe Biden's Super Tuesday event in Los Angeles, California EPA Super Tuesday: Lively scenes as results flood in across 14 states Supporters of Bernie Sanders cheer as results are announced at a Super Tuesday party in Texas AFP/Getty Super Tuesday: Lively scenes as results flood in across 14 states Supporters of Joe Biden hold up an election pooster from Barack Obama's 2008 run at a Super Tuesday event for the Democratic contender Reuters Super Tuesday: Lively scenes as results flood in across 14 states Bernie Sander takes to the stage before supporters during his Super Tuesday event in Vermont EPA Super Tuesday: Lively scenes as results flood in across 14 states Supporters of Bernie Sanders cheer as results are announced at a Super Tuesday party in Texas AFP/Getty Super Tuesday: Lively scenes as results flood in across 14 states Supporters of Bernie Sanders cheer as results are announced at a watch party in Texas AFP/Getty Super Tuesday: Lively scenes as results flood in across 14 states Joe Biden gestures to suppporters at a Super Tuesday event for the Democratic contender AP Super Tuesday: Lively scenes as results flood in across 14 states Elizabeth Warren waves to supporters at a rally in Michigan as results come in following Super Tuesday Getty Super Tuesday: Lively scenes as results flood in across 14 states A spray painted mural of presidential candidate Senator Bernie Sanders on the side of a building in Kirby, Vernont EPA Super Tuesday: Lively scenes as results flood in across 14 states Supporters of Bernie Sandes cheer at a Super Tuesday rally in Vermont Getty Super Tuesday: Lively scenes as results flood in across 14 states Supporters of Elizabeth Warren hold sings and cheer in Cambridge Getty Super Tuesday: Lively scenes as results flood in across 14 states Democratic presidential candidate Elizabeth Warren leaves the voting booth at the Graham & Parks School in Cambridge, Massachusetts Getty Super Tuesday: Lively scenes as results flood in across 14 states A poster for sale at a rally for Bernie Sanders on Super Tuesday in Vermont Getty Super Tuesday: Lively scenes as results flood in across 14 states Democratic voter Elliot Zaagman wears a protective mask as he poses for a photo after casting his ballot in Bangkok, Thailand EPA Super Tuesday: Lively scenes as results flood in across 14 states Daisy, a Golden Retriever, outside a polling in San Diego. 1,357 Democratic delegates are at stake as voters cast their ballots in 14 states and American Samoa on what is known as Super Tuesday Getty Super Tuesday: Lively scenes as results flood in across 14 states Democratic presidential candidate Bernie Sanders (greets fellow voters after casting his ballot in his state's primary election at the Robert Miller Community Center in Burlington, Vermont Getty Super Tuesday: Lively scenes as results flood in across 14 states A first time voter stands behind a voting booth in a polling location for the North Carolina primary Reuters Super Tuesday: Lively scenes as results flood in across 14 states Democratic presidential candidate Michael Bloomberg takes part in his Super Tuesday night rally in West Palm Beach, Florida Reuters Super Tuesday: Lively scenes as results flood in across 14 states Fourteen states are holding their primaries with more than one third of the total pledged delegates in the Democratic primaries to be awarded on Super Tuesday Getty Images Super Tuesday: Lively scenes as results flood in across 14 states Presidential candidate Elizabeth Warren and her husband Bruce Mann greet supporters as they walk to a polling site to vote on Super Tuesday in Cambridge, Massachusett EPA Super Tuesday: Lively scenes as results flood in across 14 states Voters cast their ballots at a polling location inside an elementary school on Super Tuesday in Minneapolis, Minnesota EPA Super Tuesday: Lively scenes as results flood in across 14 states Democrat Jamie Wilson displays a sticker after voting in the Super Tuesday primary at John H. Reagan Elementary School in the Oak Cliff section of Dallas, Tuesday, March 3, 2020. (AP Photo/LM Otero) LM Otero AP Super Tuesday: Lively scenes as results flood in across 14 states Democratic voter Le'ana Freeman poses for a photo after casting her ballot at a polling station in Bangkok EPA Super Tuesday: Lively scenes as results flood in across 14 states Voters arrive to cast their ballots at a polling location inside Hunter House at Nottoway Park in Vienna EPA Super Tuesday: Lively scenes as results flood in across 14 states Democrat Jamie Wilson gets a sticker after voting in the Oak Cliff section of Dallas AP Super Tuesday: Lively scenes as results flood in across 14 states A Polling Place sign in the border town of Hidalgo, Texas EPA Super Tuesday: Lively scenes as results flood in across 14 states A girl carries her mothers ballot to the table where she will fill in her choice at the Taylor Elementary School polling location in Arlington Getty Images Super Tuesday: Lively scenes as results flood in across 14 states A man wearing an 'I Voted' sticker and a 'Bernie Abroad' badge after voting in the American presidential primary in Oxford, England Getty Images Super Tuesday: Lively scenes as results flood in across 14 states Stickers for people who vote AFP via Getty Images

Asked about Governor Gretchen Whitmer’s endorsement of Biden on Thursday, Sanders sounded miffed. “Well, that wasn’t her thoughts when I came here to help her get elected, as a matter of fact,” he said.

Speaking earlier on Friday with reporters in Phoenix, Sanders also condemned Biden’s votes as a senator on trade agreements, while brandishing new attacks over Biden’s record on federal funding for abortions, same-sex marriage and the former “don’t ask, don’t tell” policy.

But in a sign of the careful approach both candidates are taking while seeking to draw distinctions between their campaigns, Sanders reiterated multiple times that he would support Biden if he eventually won the presidential nomination, and, pressed by a reporter, said he believed Biden could beat Trump in November.

“I think we are the stronger campaign to defeat Donald Trump, but you have not heard me say that I think that Biden cannot defeat Trump,” Sanders said. “I will certainly do everything I can if he is the nominee.”

Speaking by phone to attendees at a fundraiser in Bethesda, Maryland, on Friday night, Biden said: “What we can’t let happen is let this primary become a negative bloodbath. I know I’m going to get a lot of suggestions on how to respond to what I suspect will be an increasingly negative campaign that the Bernie Brothers will run. But we can’t tear this party apart and reelect Trump.”

The Biden campaign sent one of its most high-profile new surrogates to Michigan, Sen. Amy Klobuchar, but she was far less visible than Sanders. She toured the backstage areas of an MGM Grand hotel and casino with union leaders, but the event was off-limits to out-of-town reporters.

Biden plans to visit Michigan on Monday, but Sanders – who cancelled an appearance in Mississippi – intends to campaign intensely in the state, visiting Dearborn, Flint, Grand Rapids and Ann Arbor.

Sanders also offered a fresh critique on Friday of Biden’s record on the Hyde Amendment, which bans federal funding for most abortions (EPA)

After other major candidates quit the Democratic primary race this week, Sanders has relentlessly and repeatedly contrasted himself with Biden.

“As we enter the moment in this campaign where we come down to a two-person race, I think it important for us to differentiate our records, and I intend to do that,” Sanders said in Phoenix.

“Joe has been around for a long time; so have I. And I think people would want to take a look at those records,” he added. “All I can tell you, whether it’s Iraq, whether it’s DOMA, whether it’s ‘don’t ask, don’t tell’ – those were difficult votes. I was there, on the right side of history, and my friend Joe Biden was not.”

He was referring to the Defence of Marriage Act, which defined marriage as a union between a man and a woman, and the policy that prohibited openly gay people from serving in the military.

As a senator, Biden supported a broad defence bill in 1993 that included the “don’t ask, don’t tell” policy but also voted for a failed amendment that would have removed the policy from that bill. He also voted for the Defence of Marriage Act in 1996.

But as vice president, he said in an interview on Meet the Press in 2012 that he was “absolutely comfortable” with same-sex marriage, a stance that was ahead of the Obama White House’s policy at the time.

Sanders also offered a fresh critique on Friday of Biden’s record on the Hyde Amendment, which bans federal funding for most abortions. Until June of last year, Biden had supported the measure, which critics say disproportionately affects poor women and women of colour. He reversed his position after sustaining attacks early in his campaign.

“At a time when women are under severe political assault,” Sanders said, “I think people all over this country – women and men – are going to look at the record of which candidate has been consistent and strong in terms of defending a woman’s right to choose. On this issue, Joe Biden repeatedly voted for the Hyde Amendment.”

Sanders has watched his delegate lead evaporate after the first three nominating contests as Biden racked up wins in South Carolina and several Super Tuesday states, including some that Sanders had hoped would wind up in his column.

Moving forward, the two candidates will essentially go head-to-head next week in six states including Michigan, which will present one of their first tests in a major general-election swing state.

On Friday, Sanders, who is Jewish, also addressed an episode at a rally for his campaign in Phoenix on Thursday, when a man in the crowd unfurled a flag bearing a swastika before he was removed.

“It is horrific,” Sanders said. “It is beyond disgusting to see that in the United States of America there are people who would show the emblem of Hitler and Nazism. And I was shocked to learn about that.”

Separately, Sanders acknowledged that his team had communicated with the federal authorities about the possibility of Secret Service protection for him. Some Democratic lawmakers have called for added security after Biden’s Super Tuesday address was disrupted by protesters who were able to gain access to the stage.