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Former Vice President Joe Biden and Vermont Sen. Bernie Sanders looked for a win in Michigan on Tuesday to prove they have the backing of working-class voters in a critical Midwestern battleground state that helped put President Donald Trump in the White House.

But whereas a victory would simply pad the lead of a suddenly confident Biden fresh off a Super Tuesday rout, a loss could hammer a Sanders campaign that is struggling to find big wins in the upcoming Democratic presidential primary states. Michigan is the biggest prize on a night when Missouri, Mississippi, North Dakota, Idaho and Washington state all vote.

“You’re the best damn workers in the world,” Biden shouted through a megaphone while touring a Detroit auto plant as workers in hard hats chanted, “Joe! Joe!”

Biden now frequently ticks off the names of six former presidential rivals who have endorsed him just in the past week, saying he is “the candidate that they think can win.” The former vice president has campaigned in recent days with two of them, Sens. Cory Booker and Kamala Harris, and appeared with Michigan Gov. Gretchen Whitmer. All three have have been mentioned as possible vice presidential picks.

It wan’t all good feelings, though. While rallying at the auto plant, Biden was interrupted repeatedly by protesters angered by Biden’s support for the North American Free Trade Agreement and reluctance to embrace sweeping environmental proposals outlined in the Green New Deal. In a scuffle with demonstrators, Biden senior adviser Symone Sanders was knocked to the ground but was unhurt. Less than a week ago, she tackled a protester who rushed the stage as Biden spoke in California.

Biden also endured a testy exchange with a worker who accused him of “actively trying to end our Second Amendment right.” Biden responded, “You’re full of shit,” and went on to say that while he supports the Second Amendment, “Do you need 100 rounds?” Biden’s gun control plan reinstates the assault weapons ban and includes a voluntary buyback program for assault weapons, stopping short of a mandatory buyback program that some of his opponents had supported in the primary.

Trump won Michigan by only about 10,000 votes in 2016. That was even closer than Pennsylvania or Wisconsin, which, along with Michigan, are credited with handing the president a narrow Electoral College victory even as Hillary Clinton clinched the popular vote.

Sanders, who added credibility to his insurgent 2016 primary challenge of Clinton with a win in Michigan, has predicted he’ll emerge victorious there on Tuesday. If he doesn’t, though, he might be relegated to the role of simple protest candidate as Biden piles up a wide lead in delegates to the Democratic National Convention this summer in Milwaukee.

Although he has rejected notions he could drop out of the race if Tuesday goes badly, Sanders was visiting polling stations in Detroit on Tuesday, scrounging for late-breaking supporters. He’s said he’s now battling the “Democratic establishment” and scoffed at suggestions that so much of the party’s elite supporting his opponent means Biden is more electable.

“In a general election, which candidate can generate the enthusiasm and the excitement and the voter turnout we need?” Sanders asked. “If you want to defeat Trump, which all Democrats do and the majority of independents do and some Republicans do, we are that campaign.”

Detroit neighbors Fayette Turner and Margaret Marsh were split on which to support: Turner voted for Sanders, while Marsh voted for Biden. But they agreed on one thing.

“Anybody but Trump,” Turner, 64, said. Marsh, 69, said her family has identified as Republican her entire life — until Trump took office.

“The people I like best, they’re all gone,” Marsh said. “I think Biden’s the sanest one left. Hopefully he’ll have a good vice president.”

Voting didn’t go smoothly everywhere. Early Tuesday morning, the mayor of Kansas City, Missouri, tweeted that he had been turned away from voting because “I wasn’t in the system, even though I’ve voted there for 11 years.”

Sanders is optimistic about Washington state, Idaho and North Dakota, but Biden is looking for a strong finish in Missouri and Mississippi, where he could continue his success among African American voters. Michigan is vital because next week features a primary in Florida that is likely to dominate coverage and where Sanders’ past defense of Fidel Castro looms large — likely good news for Biden.

“If Sanders wins again, that could recast the race yet again. Then he can say, ‘I won a battleground state, a state that’s going to be key to the election,’” said Mark Brewer, a former longtime chairman of the Michigan Democratic Party. “If Biden wins, he can say the same thing.”