Louisiana on Friday became the first state in the United States to postpone a scheduled presidential primary election because of the ongoing coronavirus outbreak.

Voters in Louisiana were due to vote on April 4 in both Democratic and Republican primaries to select candidates for the November general election. Secretary of State of Lousiana Kyle Ardoin said the vote will now take place on June 20.

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A spokeswoman for the Democratic governor of the state, John Bel Edwards, said state law allows such measures to be taken in the event of an emergency. She called the move extraordinary, but prudent.

"We’re one of the few states that is supposed to have an election in early April, which we think could potentially be the height of some of this in Louisiana," spokeswoman Christina Stephens said. "Our poll workers are by large elderly - over the age of 70 - and we think it is unsafe for them to be monitoring the election. We don’t think we would have enough poll workers ... and we think we should be discouraging people from congregating in that way."

As of Friday morning, Louisiana had tested 94 people with 33 "presumptive" positive results for the coronavirus.

Observers said they other states may follow Louisiana's lead. Florida, Ohio, Arizona, and Illinois are scheduled to hold primary elections next week on March 17, and Georgia is scheduled to hold its vote on March 24. Several other key states, among them New York and Pennsylvania, have primaries at the end of April.

In a joint statement issued Friday, the secretaries of state of the four states voting March 17 insisted that the vote will go forward and that voters "can be confident that voting is safe".

"Americans have participated in elections during challenging times in the past, and based on the best information that we have from public health officials, we are confident that voters in our states can safely and securely cast their ballots in this election, and that otherwise healthy poll workers can and should carry out their patriotic duties on Tuesday," the statement said.

The pandemic already has irrevocably altered the face of the 2020 presidential campaign, with rallies and in-person events - so-called "retail" politicking - cancelled by both Democratic candidates and President Donald Trump.

Former Vice President Joe Biden, currently the frontrunner for the Democratic nomination, has instructed all campaign employees across the country to work from home beginning Saturday. The edict will remain in effect for at least two weeks, the campaign said.

In a memo to staff, campaign officials said the Biden campaign may continue to hold smaller events unless current guidance from public officials changes, but fundraisers would become "virtual" indefinitely.

The campaign of challenger Bernie Sanders issued the same orders to its staff on Thursday. Future events would be evaluated for safety on a case-by-case basis, the campaign said.

After initially hesitating, the president’s campaign officials said they were cancelling his trademark rallies, as well as all fundraisers and other events, for at least the next week. Trump was slated to fly west on Thursday night for events in Nevada and Colorado, a trip that was cancelled after his Oval Office address to the nation on Wednesday night about the severity of the coronavirus outbreak.

The Trump campaign also canceled a "Catholics for Trump" coalition event, which Trump planned to attend, that had been scheduled for Milwaukee next week. Vice President Mike Pence also planned to stop all campaign activity for two weeks, a White House official said.