In Flint, the fiery Harvard professor Cornel West, a Sanders surrogate, wound up the crowd by accusing Mr. Biden of being “a neoliberal,” eliciting hearty boos. Afterward, an audience member, Bob Granville, echoed the critique. Neoliberalism, he said, “means socialism for the rich and a rugged capitalism for everyone else.”
If Mr. Biden is the nominee, said Mr. Granville, a retired work-force trainer, he plans to vote for a third-party candidate in protest. “People say, oh, you’re going to be part of letting Trump in there. I say, if you’re going to run Biden, we deserve Trump.”
Despite the avowed desire of the Sanders campaign to broaden his appeal, Mr. Sanders himself has a penchant for pushing away moderate voters he needs by frequently attacking the Democratic “establishment.” On Sunday, he accused party leaders of forcing Ms. Klobuchar and Pete Buttigieg, the former mayor of South Bend, Ind., out of the race to help Mr. Biden “and try to defeat me.”
David Betras, a former Democratic chairman of Mahoning County, Ohio, in an industrial region where Mr. Trump did well in 2016, said he feared that the more Mr. Sanders attacks “the establishment,” the more enraged his supporters will become, with many likely to sit out the general election. “His supporters are fervent and will feel betrayed by Democrats.”
That is not an inevitability. Mr. Sanders explicitly tells supporters in nearly every speech that he will back Mr. Biden as the nominee.
And there are anecdotal signs that Mr. Sanders may be growing support.
Eric Mays, a city councilman in Flint, arrived as an undecided voter to his rally there on Saturday, which was billed as “a town hall on racial and economic justice.”
“Bernie has got me fired up,” he said. “I might very well be voting Bernie on Tuesday.”
Mr. Mays said that despite the overwhelming support of black voters in Southern states for Mr. Biden, Michigan could break differently. “We know Bernie Sanders will get black voters out of Flint,” he said.
“I hope that we see him all the way through this year.”