Louisiana on Friday became the first US state to postpone its Democratic presidential primary in a bid to stem the rapidly spreading coronavirus pandemic.
With schools closing nationwide and an increasing number of Americans staying home, the Deep South state said the vote would go ahead on June 20 -- 11 weeks after the planned date of April 4.
"The law provides for extraordinary measures to be taken when facing disasters," Louisiana Secretary of State Kyle Ardoin said.
The decision comes with the race for the party's presidential nomination undecided, days before voting in Ohio, Florida, Arizona and Illinois -- which have all reported infections.
Millions were expected to choose between former vice president Joe Biden and Senator Bernie Sanders, who are vying for the chance to face Republican President Donald Trump in November's election.
Confirmed coronavirus cases in the US topped 1,700 on Friday, with 41 deaths, according to figures from Johns Hopkins University and the Centers for Disease Control.
Ohio, which has five confirmed cases, declared a statewide emergency last week but has not put off its primary.
Louisiana also delayed until June 20 its Republican primary, a ceremonial affair, with Trump is the only viable candidate.
Biden leads the Democratic race, with 878 delegates over Sanders' 725. To win the nomination, a candidate needs a majority of 1,991.
Both candidates have curtailed campaigning, canceling rallies in Ohio, Illinois and elsewhere, and telling staffers to work from home.
Biden, who is reliant the older demographic that is more at risk from the outbreak, is holding online 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.
"If voters are members of an at-risk population, exhibiting symptoms, or have been exposed to a diagnosed case of COVID-19, we encourage them to explore absentee ballots and vote by mail options."
Both candidates have attacked Trump's handling of the coronavirus crisis.
"In terms of potential deaths and the impact on our economy, the crisis we face from coronavirus is on the scale of a major war, and we must act accordingly," Sanders said Thursday.
"Unfortunately, in this time of international crisis, it is clear to me at least that we have an administration that is largely incompetent, and whose incompetence and recklessness have threatened the lives of many, many people in our country."
