The Democratic presidential primary has become shrouded in uncertainty after Ohio called off Tuesday’s election just hours before polls were set to open due to the coronavirus.

Officials in Florida, Arizona and Illinois, however, said they would move forward with the vote as planned.

Not since New York City postponed its mayoral primary on the day of the September 11, 2001, attacks has an election been postponed in such a high-profile way.

Ohio governor Mike DeWine initially asked a court to delay the vote. When a judge refused to do so, the state’s health director declared a health emergency that would prevent the polls from opening.

Virus Outbreak US Primaries
A voter fills out his ballot, early in Steubenville, Ohio, on Sunday, a day before the Democratic primary there in the state was called off (Gene J Puskar/AP)

The contests are playing out as the impact of the virus becomes more tangible, with schools closing across the country, workers staying home and restaurants and bars shutting.

The rapidly shifting developments amounted to a kind of chaos rarely seen in an election season. And it may not end soon as some states that have presidential contests in the coming weeks have already moved to postpone them and others were being pressed to follow.

Campaign workers spent Monday sifting through data and talking to contacts on the ground to assess the impact of the coronavirus on turnout in places that will hold elections on Tuesday.

Former Vice President Joe Biden is moving closer to securing the Democratic presidential nomination, but could face a setback if the older voters who tend to support him do not show up.

His rival, Vermont senator Bernie Sanders, meanwhile, can not afford to lose support from young voters, who have been his most loyal supporters.

The tumult has left the campaign in a state of suspended animation. In-person rallies have been replaced with sometimes-awkward virtual events.

Mr Sanders, the last Democrat standing between Biden and the nomination, is not planning to drop out.

His campaign looked to have nowhere to go after a sizeable loss last week in Michigan, and another blow landed on Monday night when Mr Biden was declared the winner of the primary in Washington state, giving him victories in five out of six states that voted last Tuesday.

Yet Mr Sanders’s top advisers see no downside to staying in the race as they assess how the coming days and weeks unfold.

Mr Sanders staged a virtual rally Monday night featuring himself, rocker Neil Young and activist actress Daryl Hannah.

He also released a video criticising Mr Biden for suggesting as a senator that he would be willing to cut Social Security benefits — a line of attack he employed frequently during Sunday’s debate.

“I don’t have to tell anybody that we are living in a very unprecedented and strange moment in the history of our country,” Mr Sanders said, urging supporters that it may be time to “rethink our value system, rethink many of the systems we operate under”.

Mr Sanders’ team had expected Mr Biden to do well in all four states set to vote on Tuesday.

But the Vermont senator has also cast some doubt about the entire process, saying no one should risk being infected while voting and noting that it was important “to make sure that everybody who wants to vote has the right to vote, and that may not be the case now”.

Still, Mr Sanders faces an increasingly tough path to the nomination.

About half of the delegates in the Democratic primary have already been awarded and, if Mr Biden has another big night on Tuesday, he will extend an already large and perhaps insurmountable lead.

Mr Sanders trails Biden by more than 150 delegates nationally, meaning he would need to win more than 57% of those yet to be allocated to clinch the Democratic nomination.

Mr Biden appeared to keep his focus Monday on winning the nomination, as he encouraged voters in a telephone town hall to participate in Tuesday primaries but to do so safely.