Ohio called off its presidential primary just hours before polls were set to open there and in three other states, an 11th-hour decision the governor said was necessary to prevent further fueling the coronavirus pandemic that has paralysed the nation.

Health Director Amy Acton declared a health emergency that would prevent the polls from opening out of fear of exposing voters and volunteer poll workers - many of them elderly - to the virus.

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Governor Mike DeWine failed to get a judge to halt the primary on Monday evening, even though the governor contended the election results wouldn't be viewed as legitimate in light of the pandemic.

"To conduct an election tomorrow would would force poll workers and voters to place themselves at a unacceptable health risk of contracting coronavirus," he said. 'We cannot tell people to stay inside, but also tell them to go out and vote," he added.

It wasn't clear what would happen, but DeWine said officials were considering how to give voters an opportunity to cast their ballots.

Arizona, Florida and Illinois were proceeding with their presidential primaries.

In Illinois, there is no process for cancelling or postponing the vote, said Matt Dietrich, a spokesman for the State Board of Elections.

"This is unprecedented, so it's not clear exactly what the process would be for changing it, though it likely would involve a request to the attorney general to seek a court order," Dietrich said.

So far, three other states - Georgia, Louisiana, and Kentucky - have said they will postpone their primary elections. Georgians were scheduled to vote on March 24, voters in Louisiana on April 4, and those in Kentucky on May 19. The states will now vote in May and June.

The candidates themselves have questioned the wisdom of going through with the elections. In an interview with CNN following Sunday night's debate, Democratic presidential candidate Bernie Sanders said he hoped the governors would listen to health experts about the dangers of mingling with others in public spaces.

"I'm thinking about some of the elderly people who are sitting behind the desks, registering people, doing all that stuff," he said. "Does that make a lot of sense? Not sure that it does."

The coronavirus outbreak has dramatically upended normal presidential campaign routines, forcing both remaining Democratic presidential candidates off the road. Both Sanders and frontrunner Joe Biden plan to hold "virtual" campaign events on Monday in an effort to reach voters.

Biden will hold a tele-town hall meeting with voters in all four states on Monday, his campaign said, while Sanders planned a "digital rally" livestreamed broadcast featuring musician Neil Young and actress Daryl Hannah.

State officials said they were taking extra precautions to protect the public on Tuesday. In Florida, polling places located in nursing homes and assisted living facilities have been moved to avoid exposing the residents to outsiders.

Florida's Broward County is stocking its 421 polling locations with extra supplies, including 4,000 rolls of paper towels, gloves and more than 400 bars of soap.

"We've purchased gallons of rubbing alcohol and are having them transferred to spray bottles," said spokesman Steve Vancore. "We've ordered cases upon case of Clorox wipes so poll workers can frequently wipe down the equipment and wipe down the voting booths."

The Democrats' presidential nomination fight has been relegated to an afterthought as the US grapples with a health crisis likely to grow exponentially in the coming weeks.

If the primaries are held as scheduled, the vote Tuesday could spell a death knell for Sanders, who cannot afford a repeat of last week's performance, when he lost four of the six states to Biden. Sanders also looks likely to lose in Washington state, where votes are still being counted.

Florida, with 219 of the 1,991 delegates needed to secure the nomination, is the big prize of the day, but Sanders has struggled in that state with both seniors and some older Latinos, who may harbour ill will towards anyone associated with socialism because of experiences in places such as Venezuela and Cuba. A bad night in Florida could make Sanders's path to the nomination all but impossible.