Americans voted Tuesday in Democratic primaries in Florida, Illinois and Arizona in an election day like no other, as the rapidly spreading coronavirus pandemic blighted the US presidential nomination race between frontrunner Joe Biden and Bernie Sanders.

A fourth state scheduled to vote Tuesday, Ohio, postponed its elections until June on orders of Republican Governor Mike DeWine, who told voters to stay at home amid the escalating public health emergency.

Tourists wear masks as they visit the Universal Studios theme park in Orlando, Florida, a state that is holding its Democratic presidential primary on March 17, 2020 despite widespread coronavirus outbreaks
Tourists wear masks as they visit the Universal Studios theme park in Orlando, Florida, a state that is holding its Democratic presidential primary on March 17, 2020 despite widespread coronavirus outbreaks AFP / Susan STUMME

The US has officially registered more than 5,600 coronavirus cases, although that is believed to be a fraction of the real number, which has been depressed by delays in rolling out testing.

Election authorities are braced for disruptions with President Donald Trump's administration urging Americans to avoid gatherings of more than 10 people.

Trump backed the Ohio delay at a White House news conference, telling reporters asking what he was doing to protect elections: "What I am doing is getting rid of the virus. That's the only thing we can do."

Democratic presidential hopefuls former US vice president Joe Biden (L) and Senator Bernie Sanders greet each other with a safe elbow bump before the start of the Democratic presidential debate on March 15, 2020
Democratic presidential hopefuls former US vice president Joe Biden (L) and Senator Bernie Sanders greet each other with a safe elbow bump before the start of the Democratic presidential debate on March 15, 2020 AFP / Mandel NGAN

In Florida, the largest and most populous state voting Tuesday, some two million people had already voted early or by mail, and in-person turnout was expected to be exceptionally low.

Adding to obstacles, state authorities had to move dozens of precincts from senior centers to protect against the virus.

Gabriela Carrilho, a 51-year-old who works in marketing, donned gloves before casting her ballot at a nearly deserted elementary school in Miami Beach.

Election workers hand out "election delayed" signs to put up at polling stations in Dayton, Ohio -- the state's governor says the primary will be held at a later date
Election workers hand out "election delayed" signs to put up at polling stations in Dayton, Ohio -- the state's governor says the primary will be held at a later date AFP / Megan JELINGER

"I think if you don't participate, then things never change," she said.

She and others said they felt safe voting.

"There was hand sanitizer at every single station," said 29-year-old Matt Don. "They kept a safe distance from me. Everybody in there is keeping safe."

Map showing the three US states voting in Democratic presidential primaries on March 17, 2020, with states and delegates won so far by Joe Biden and Bernie Sanders
Map showing the three US states voting in Democratic presidential primaries on March 17, 2020, with states and delegates won so far by Joe Biden and Bernie Sanders AFP / Laurence SAUBADU
Joe Biden and Bernie Sanders have said they will campaign exclusively online for now due to the coronavirus crisis
Joe Biden and Bernie Sanders have said they will campaign exclusively online for now due to the coronavirus crisis GETTY IMAGES NORTH AMERICA / MANDEL NGAN

In Illinois, Chicago election commissioners scrambled to find substitute voting judges after hundreds informed the county clerk's office they wouldn't show up.

"This is the biggest test that any election jurisdiction has faced in the last hundred years," Chicago Board of Elections head Jim Allen said of voting during the pandemic.

The board told reporters that voter turnout was low.

Chicago voters cast ballots during Illinois primary in Chicago, Illinois, on March 17, 2020
Chicago voters cast ballots during Illinois primary in Chicago, Illinois, on March 17, 2020 AFP / KAMIL KRZACZYNSKI

But Governor J.B. Pritzker refused to bend rules and postpone the election, even as he announced the state's first confirmed death from coronavirus, saying "the constitutional boundaries of our democracy" should be respected.

"If people want to criticize me for that," he said. "I'll wear it like a badge of honor."

Voters also trickled in to cast ballots in Arizona, where polls close a few hours after those in Florida and Illinois. Results could start coming in shortly after 8:00 pm (0000 GMT Wednesday).

A potential scare factor for the elderly, who are at highest risk of contracting COVID-19, could be at play and is expected to suppress in-person turnout.

Democrats are choosing a nominee who will challenge the Republican incumbent in November's election, but doubts and concerns have undercut the process.

"While Arizona, Florida and Illinois are still voting today, going to the polls amid the coronavirus outbreak is a personal decision and we respect whichever choice voters make," Sanders tweeted.

Biden urged voters to take precautions such as social distancing at polls.

"If you vote in person, please wash your hands, don't touch your face, and stay 6' from others in line," he urged.

Biden tops polling by significant margins in Arizona, Illinois and especially Florida, where the 77-year-old could strike a hammer blow against Sanders by building an insurmountable lead in the delegate count.

The leftist Sanders faces a make-or-break moment -- and mounting pressure to quit if he fails to score a major upset.

Meanwhile, Americans are staying home from work or school by the millions as the country implements emergency measures against a worsening crisis.

Maryland on Tuesday became the latest state to delay voting, pushing its primary from April 28 to June 2.

Kentucky and Louisiana have postponed their elections to June, while Georgia is delaying from next week until May.

The Democratic National Committee urged the remaining states to expand voting tools including vote by mail and extended days of early voting.

"The right to vote is the foundation of our democracy, and we must do everything we can to protect and expand that right instead of bringing our democratic process to a halt," DNC chairman Tom Perez said.

Biden and Sanders have been left to campaign exclusively online for now -- a scenario unheard of at the height of a US primary battle.

Sanders hosted a "digital rally" Monday night, when he acknowledged he was the underdog but stressed his progressive movement has "transformed" the 2020 campaign by convincing rivals like Biden to support some of his liberal policies.