DETROIT (AFP) -- Joe Biden took another big step closer to clinching the Democratic nomination to challenge President Donald Trump, with wins projected Tuesday over leftist rival Bernie Sanders in Mississippi and Missouri, two of six states holding primaries.

The Mississippi win, projected by US networks, confirmed crucial and overwhelming support from African American voters for Biden, the former vice president under the first black US president, Barack Obama.

Results from four more states, including the key battleground of Michigan, were due in shortly, with Biden looking to build an invincible lead in what has become a two-man race with the self-proclaimed democratic socialist Sanders.

Adding to nerves in a country on edge after three tumultuous years under Trump, fears of the coronavirus epidemic prompted both campaigns to cancel election-night rallies.

Organizers of a live television debate scheduled Sunday, pitting the fiery Sanders and the veteran centrist Biden, likewise said they would take the extraordinary step of not allowing the usual live audience.

But for Biden, the six-state contest, coming a week after his surprise comeback in voting on Super Tuesday, was a chance to demonstrate that his once dismissed campaign is in rude health.

Michigan in particular -- an industrial giant that Trump won in an upset in 2016 -- was being targeted by Biden as a place to put down a marker ahead of the November general election.

Sanders has also acknowledged that Michigan is critical as a test of his message of revolution against the economic status quo.

Biden opened primary day meeting workers at an under-construction automobile plant in Detroit, where he received cheers but also was confronted by one worker.

In an exchange avidly shared online by Trump supporters, the worker, wearing a construction helmet and reflective vest, accused Biden of seeking to take away Americans' firearms.

"You're full of shit," an angry Biden shot back, insisting that he supports the constitutional right to bear arms.

When the worker pressed the issue, Biden said with a raised voice.

"I'm not taking your gun away," he said, adding, "Gimme a break, man."

Coronavirus fears

Voters came out under the growing shadow of the global coronavirus epidemic, which has infected about 900 people across the United States and killed 28.

Sanders and Biden called off customary election-night rallies -- both planned in Cleveland, part of another Rust Belt state, Ohio, which votes next week -- in line with public health warnings from state officials.

Their precaution is at odds with Trump, who has vowed to keep holding his campaign's sometimes raucous rallies despite the concerns.

Washington state, which has borne the brunt of the crisis and also was going to the polls Tuesday, votes entirely by mail -- an option some experts say should become more widespread as the epidemic grows.

Idaho and North Dakota were also voting.

But the day's top prize is Michigan, with its large delegate haul, status as a swing state and diverse demographics that mirror the United States.

'Most dangerous' president

Artist and designer Cecilia Covington, 61, was the first person to vote in Precinct 123 in downtown Detroit, braving the drizzle as she arrived at Chrysler Elementary School to vote for Biden.

"When he wasn't doing well in the polls I was really concerned," she said, adding that his stunning comeback on Super Tuesday "put my confidence back."

"We've got to get '45' out of office," she said, referring to the current president.

Sanders has highlighted Biden's support for free-trade deals to win over Michigan's blue-collar voters, many of whom ditched the Democrats for Trump four years ago.

"We are in a crisis in America, not only in the need to defeat Donald Trump, the most dangerous president in modern American history, but to take on the greed and corruption of corporate elite," Sanders said in an ABC interview.

"That is what our campaign is about. It's very different than Joe's."

Biden narrowly leads delegates

Biden has also courted blue-collar workers, highlighting his own hard-knock childhood in Pennsylvania and Delaware.

"Wall Street didn't build America, you built America," Biden told a rally in Detroit. "Unions built the middle class."

Biden won 10 out of 14 states that voted on Super Tuesday and holds a narrow lead in delegates to pick the nominee, although Sanders carried the most populous state of California.

Trump has sought to portray Sanders as the victim of a conniving Democratic establishment, a narrative that some experts believe could depress turnout and benefit Republicans.

"Going to be a BAD day for Crazy Bernie!" Trump tweeted.

But Sanders supporter Frank Anderson, a 24-year-old software engineer, said he was optimistic for a victory, noting how Sanders triumphed in the 2016 Michigan primary over Hillary Clinton.

"They had counted us out then, so I'm hopeful for today," Anderson said. (AFP)