Joe Biden and Bernie Sanders met on Sunday for the first one-on-one debate of the Democratic presidential primary campaign, now overshadowed by the spread of the novel 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. Trump took a COVID-19 test on Friday and was cleared of the illness on Saturday by his physician, after meeting with members of a Brazilian delegation who later tested positive. 

The two would 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. 

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

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.

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Meanwhile, Biden has won the backing of the National Education Association (NEA), the largest U.S. teachers union with three million members. 

"Joe is the tireless advocate for public education and is the partner that students and educators need in the White House," NEA President Lily Eskelsen Garcia said in a statement on Saturday. 

"He understands that we have a moral responsibility to provide a great neighborhood public school for every student in every ZIP code." 

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The former U.S. vice president, whose wife, Jill Biden, is a teacher and NEA member, said he was honored to get the endorsement of a "powerful voice for public school educators and students across the country." 

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 Trump. 

(With input from AFP, Reuters)