In a day of primary elections described as surreal by some observers, former Vice President Joe Biden all but cemented his status as the presumed Democratic nominee to challenge President Donald Trump in the November general election.

Biden swept all three states in voting on Tuesday - Illinois, Florida, and Arizona - in another blow to Vermont Senator Bernie Sanders - whose early strength evaporated as African Americans and working-class whites across the country sided with Biden.

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In the big prize of the evening, Florida, election-watchers called the state for Biden almost immediately after the polls closed at 7pm local time (23:00 GMT). Illinois was called for the former vice president within 30 minutes of the polls closing there and Arizona was handed to Biden almost as quickly.

In an online speech from his home in Wilmington, Delaware after the results, Biden made an explicit appeal to the younger voters who had flocked to Sanders.

"I hear you," Biden said. "I know what’s at stake. I know what we have to do. Our goal as a campaign, and my goal as a candidate, is to unify this party and unify this nation."

Ohio had been scheduled to vote on Tuesday as well, but officials in that state took the unprecedented step of ignoring a judge's order to carry on with the election and shut down polling centres out of fears over the coronavirus pandemic. The move followed similar ones in Georgia, Maryland, Louisiana, Ohio and Kentucky, all of which have postponed upcoming primaries because of the crisis.

Democratic party officials criticised the moves, saying state officials should focus on expanding mail-in voting and expanding the hours of days of service in order to reduce lines and crowding instead of delaying the vote entirely.

"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," Democratic National Committee Chairman Tom Perez said in a statement.

Millions of voters had already participated in some form of early voting by the time Tuesday rolled around. But there were signs on the day that voters - and poll workers - were staying home because of the rapidly expanding coronavirus crisis, hindering the voting.

Jefferson County Elections officials arrive to pack up the polling place at Our Lady of Lourdes church in Wintersville, Ohio on Tuesday. Ohio's presidential primary was postponed on Tuesday amid coronavirus concerns. [Gene J. Puskar/AP Photo]

In Burbank, Illinois, a small community southwest of Chicago, for example, most of the voting stations stood empty at 8am local time (13:00 GMT). Only 17 people had voted, a pace that officials said was unusually slow. In Chicago itself. About 50 polling sites opened late and in Okaloosa County in Florida's Panhandle, two dozen poll workers did not turn out at the last minute, leaving election officials scrambling to find replacements.

"People are prioritising their day-to-day survival right now - so they're not thinking of voting as a priority," said Debra Cleaver, the founder of Vote.org.

Sanders, the last Democrat standing between Biden and the nomination, reportedly is not planning to drop out, with his team seeing no downside to staying in the race as they assess how the coming days and weeks unfold. Staying in, they said, would give Sanders more influence over the Democratic platform at its convention in Milwaukee, Wisconsin in July.

Instead of the customary campaign speech after the results of the vote, Sanders on Tuesday night addressed his supporters online in a 20-minute live speech in which he outlined proposals he had for addressing the coronavirus and its economic effect, including tasking the US military with building hospitals and testing facilities and paying every US household $2,000 a month until the economic disruption passes.

"We must make certain that this health and economic crisis is not another money-making opportunity for corporate America and for Wall Street," Sanders said.

Sanders, however, has an almost impossible path to the nomination at this point. About half of the delegates in the Democratic primary had already been awarded before Tuesday and Biden's big win on Tuesday gives him an all-but insurmountable lead. Sanders trails Biden by more than 150 delegates nationally, meaning he would need to win more than 57 percent of those yet to be allocated to clinch the Democratic nomination.

Jesse Lehrich, a Democratic operative and former Hillary Clinton campaign spokesman who is based in Chicago, described the vote on Tuesday as "eerie".

"Biden and Sanders are debating the merits of marginally different policies in this little pseudo-reality, while America is consumed by an unprecedented crisis," Lehrich said. "It all feels like a bizarre formality given the moment - a pointless subplot with a foregone conclusion, in the midst of an existential threat."

Also on Tuesday, Trump formally clinched the Republican Party nomination, surpassing the necessary delegate threshold. Trump, who had only token opposition, now has more than the 1,276 delegates needed after winning Tuesday's Florida and Illinois primaries, according to The Associated Press news agency delegate count.