Joe Biden and Bernie Sanders meet on Sunday for the first one-on-one debate of the Democratic presidential primary campaign, now overshadowed by the spread of the new coronavirus.

Both men have cancelled rallies and two state contests have been postponed in the wake of the outbreak, which has killed at least 57 Americans and upended daily life across the country.

Many states and cities have clamped down on large gatherings and closed schools, which are often used as polling places, to help contain the epidemic.

Frontrunner Biden and self-described "democratic socialist" Sanders are vying to replace President Donald Trump, who was on Saturday cleared of the COVID-19 illness by his physician after meeting with members of a Brazilian delegation who later tested positive.

They will face off for two hours from 8:00 pm (0000 GMT Monday), but Democratic officials have shifted the venue from Arizona to a TV studio in Washington DC with no live audience because of infection fears.

The debate comes ahead of key electoral contests on Tuesday in Florida, Ohio, Illinois and Arizona. Officials in all four states said they would work to make conditions safe for voters.

But Georgia delayed its primary election by nearly two months on Saturday, with state Democratic Party chairwoman Senator Nikema Williams warning that the ballot risked compromising the health and safety of voters.

A day earlier, Louisiana announced it would postpone its own primary vote by 11 weeks.

Both candidates have curtailed campaigning, cancelling rallies in Ohio, Illinois and elsewhere, and telling staffers to work from home.

Biden surge

Biden, who is reliant on the older demographic that is more at risk from the outbreak, has held online campaign events and has urged voters to look for alternative ways to cast their ballot.

"If voters are feeling healthy, not exhibiting symptoms, and don't believe they've been exposed to COVID-19, please vote on Tuesday," deputy campaign manager and communications director Kate Bedingfield said in a statement.

Sanders sounded more supportive about postponing primaries without explicitly calling for a delay.

Officials must "make sure that everybody who wants to vote has the right to vote, and that may not be the case today," he said.

After a disastrous start, former vice president Biden has surged to the front of the race with a sweep of every state to vote in the American South so far.

Biden leads the overall race as well with 878 delegates over Sanders' 725. To win the nomination, a candidate needs a majority of 1,991.

In a speech Wednesday, Sanders said he was winning the "ideological debate" but acknowledged that he was "losing the debate over electability" -- that is, the all-important goal for many Democratic voters of finding the candidate best able to defeat Donald Trump.

The current consensus in the party points to Biden as the candidate best positioned to achieve that goal. 

https://www.youtube.com/watch?v=pOavNaBX4NU