Bernie Sanders will learn on Tuesday whether Michigan will revive his Democratic presidential bid campaign or relegate him to the role of protest candidate, four years after his landmark primary victory before the 2016 election.

Mr Sanders proved his last presidential bid was serious with his Michigan triumph.

The 78-year-old appealed to working-class voters and was powered by his opposition to free trade.

Michigan and five other states hold presidential contests on Tuesday at a critical point in the Democratic race.

A total of 352 delegates are up for grabs in Michigan, Mississippi, Missouri, Idaho, North Dakota and Washington, but Michigan has the most delegates - 125.

Joe Biden speaks during a campaign rally in Detroit, Michigan. Credit: AP

Former vice president Joe Biden is looking to quash Mr Sanders’ hopes and cement his own front-runner status just a week after resurrecting his beleaguered White House bid with a delegate victory on Super Tuesday.

Super Tuesday refers to the Tuesday in a presidential election year when the largest number of states and territories hold primary elections and caucuses.

He played up his underdog story on Monday as he campaigned across Michigan, reflecting on his stutter as a child and the deaths of his first wife and young daughter.

And Mr Biden was not alone.

The former vice president courted the state’s influential African American voters alongside the two most prominent black candidates previously in the 2020 race, Kamala Harris and Cory Booker.

Kamala Harris has backed Joe Biden. Credit: AP

Mr Sanders is in an urgent fight to turn things around as the primary calendar quickly shifts to other states that could favour Mr Biden.

The senator countered the parade of Democratic firepower lining up behind Mr Biden by securing the endorsement of the Reverend Jesse Jackson and deploying Representative Alexandria Ocasio-Cortez on his behalf.

Having devoted several days to Michigan, the Vermont senator spent the eve of Tuesday’s primaries in Missouri, where he downplayed questions about electability and insisted the “energy and excitement” of his supporters made him the strongest Democrat to run against President Donald Trump this autumn.

Tuesday marks the first time voters will weigh in on the Democratic contest since it effectively narrowed to a two-person race between Mr Sanders and Mr Biden.

It will be another test of whether Mr Sanders can broaden his appeal among African Americans.

Mr Biden, meanwhile, must show he can keep momentum going after his surprise Super Tuesday turnaround.

Bernie Sanders is still waiting to hear whether he will gain the endorsement of Elizabeth Warren. Credit: AP

Mr Biden ticked off the names of six former presidential rivals who have endorsed him just in the past week during a Monday appearance with Mr Booker in Flint, Michigan, a community that has seen auto industry jobs disappear.

“They’ve all come out and endorsed at one time … the candidate that they think can win," Mr Biden said.

Other economic challenges emerged on Monday as fears about the spread of the new coronavirus shook financial markets, slicing 7.8% from the Dow Jones Industrial Average.

Mr Sanders met with health experts in Detroit, and Mr Biden said he would defer to health experts about whether campaign events could continue, given concerns about the spread of the virus.

An outstanding question going into Tuesday is whether Massachusetts senator Elizabeth Warren, who dropped out of the race last week, will endorse Mr Biden or Mr Sanders.

So far, Ms Warren has declined to back either, a frustrating development for Mr Sanders, who could use her help in unifying progressives in much the same way Mr Biden has been able to rally moderates.

Mr Sanders has scoffed at suggestions he could drop out if he does not win Michigan.