US presidential hopeful Joe Biden has notched up some important wins in the latest round of Democratic primaries.
Local networks project Mr Biden beating leftist Bernie Sanders in the day's "biggest prize" of Michigan as well as Mississippi, Missouri and Idaho.
The centrist former vice president is hoping another big night in six states could make him invincible as the Democrats pick a challenger to face President Donald Trump in November.
Delegate-rich Michigan is an important electoral battleground, with a high level of working class voters who in recent years have abandoned the Democrats.
Four years ago, Mr Sanders' victory in the state hurt Hillary Clinton's chances of party nomination that ultimately assisted Mr Trump's path to victory.
Mr Biden and Mr Sanders are both septuagenarian veterans of the Senate but are running starkly different campaigns.
Joe Biden greets the crowd during a campaign rally. Source: AAP
Mr Biden, who enjoys a sizable poll lead, advocates a traditional Democratic platform of liberal legislative reforms while Mr Sanders is urging a roots-up socialist revolution.
Mr Biden opened primary day meeting workers at an under-construction automobile plant in Detroit, where he received cheers but was also confronted by one worker.
In an exchange avidly shared online by Trump supporters, the worker, wearing a construction helmet and reflective vest, accused Mr Biden of seeking to weaken the constitutional right to own firearms.
"You're full of s***," Mr Biden shot back. "I support the Second Amendment."
You're full of s*** Joe Biden
When the worker pressed the issue, Mr Biden, visibly agitated and with a raised voice, said "I'm not taking your gun away," adding, "Gimme a break, man."
Coronavirus fears
Voters came out under the growing shadow of the global coronavirus epidemic, which has infected 800 people across the US and killed 28.
Mr Sanders and Mr Biden called off customary election-night rallies - both planned in Cleveland, part of another Rust Belt state, Ohio, which votes next week - as a precaution.
"We are heeding the public warnings from Ohio state officials, who have communicated concern about holding large, indoor events during the coronavirus outbreak," said Sanders campaign spokesman Mike Casca.
Bernie Sanders on the campaign trail. Source: AAP
Their precaution is at odds with Mr Trump, who has vowed to keep holding his campaign's sometimes-raucous rallies despite the health concerns.
Washington state, which has borne the brunt of the crisis and also was going to the polls on Tuesday, votes entirely by mail - an option some experts say should become more widespread as the epidemic grows.
The Centers for Disease Control and Prevention has offered guidance on cleaning voting machines for the other states. Among them, only Missouri had confirmed a case by Tuesday according to Johns Hopkins University.
Strong black support
Artist and designer Cecilia Covington, 61, was the first person to vote in Precinct 123 in downtown Detroit, braving the pre-dawn cold drizzle as she arrived at Chrysler Elementary School to vote for Mr Biden.
"When he wasn't doing well in the polls I was really concerned," she admitted. "But when he won the South Carolina primary I put my confidence back in him."
"We've got to get '45' out of office," she said, referring to the current president.
Mr Biden - the faithful deputy for eight years to Barack Obama, the first black president - has been surging with strong backing from African Americans.
Mr Sanders has highlighted Mr Biden's support for free-trade deals to win over Michigan's blue-collar voters.
"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," Mr Sanders said in an ABC interview.
"That is what our campaign is about. It's very different than Joe's."
Biden narrowly leads delegates
Mr 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," Mr Biden told a rally in Detroit. "Unions built the middle class."
Mr Biden won 10 out of 14 states that voted on Super Tuesday and holds a narrow lead in delegates to pick the nominee, although Mr Sanders carried the most populous state of California.
Mr Trump has sought to portray Mr 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!" Mr Trump tweeted.
But Mr Sanders supporter Frank Anderson, a 24-year-old software engineer, said he was optimistic for a victory, noting how Mr Sanders triumphed in the 2016 Michigan primary over Ms Clinton.
"They had counted us out then, so I'm hopeful for today," Mr Anderson said.
"His consistency over the last 40 years, his policies aimed at, you know, restoring the middle class, and really everything he's been fighting for his entire life, I stand with," he said.
