Relatively unscathed by the growing coronavirus epidemic so far, the United States presidential campaign got a dose of reality on Tuesday when Democratic contenders Bernie Sanders and Joe Biden both cancelled major events because of the outbreak.

The Sanders campaign said it had called off a rally scheduled for Tuesday night in Cleveland, Ohio amid concerns about the virus.

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Mike Casca, a spokesman for the 78-year-old Vermont senator, said the campaign is cancelling the event out of concern for public health and safety. Sanders had been set to speak there as results from Democratic primary voting taking place in six states rolled in Tuesday night.

"We are heeding the public warnings from Ohio state officials, who have communicated concern about holding large, indoor events during the coronavirus outbreak," Casca said. "Senator Sanders would like to express his regret to the thousands of Ohioans who had planned to attend the event tonight."

A Biden spokesman initially suggested the former vice president's own rally in Cleveland would go on as scheduled, but his campaign subsequently released a statement saying that event was off as well.

On Monday, Biden told NBC News that he would heed the advice of health authorities regarding his rallies.

"I'm looking to the CDC for advice on that," Biden said, referring to the US Centers for Disease Control and Prevention. "We're going to follow the recommendations of the experts ... and if they conclude that there shouldn't be big indoor rallies, then we'll stop big indoor rallies."

Beyond the rallies, election officials in states due to hold primary contests on Tuesday and later this month are scrambling to come up with contingency plans in the event of the further spread of the virus.

Voters, some in masks, wait in line to cast their ballots in the Democratic primary at a polling station in Houston, Texas [File: Callaghan O'Hare/Reuters]

Government officials in Florida, which holds its primary March 17, urged voters to consider voting early in its contest, and Arizona - voting the same day - braced for the possibility of poll workers staying home out of fear or because they are ill. State officials are combing through a statewide volunteer database to identify potential replacements if needed, according to the Arizona Department of State.

In Cook County, Illinois, which includes the city of Chicago, officials are seeking the power to extend voting hours on March 17, reassign early voting locations and even change voting days in the event of extraordinary circumstances, according to Frank Herrera, a spokesman for the county clerk's office.

In Michigan, one of six states voting on Tuesday, officials told poll workers to sanitise voting booths and other equipment regularly throughout the day. Washington state - the hardest-hit state in the country - caught a break because its Tuesday contest is vote-by-mail.

In Mississippi, which is also voting on Tuesday, Secretary of State Michael Watson offered health tips and advice.

"We're encouraging voters to make sure they wash their hands as often as possible and continue to practice good hygiene," he said in a statement. "With no confirmed cases in our state, we obviously still want to remain cautious. We encourage all Mississippians to go vote."

On Sunday, President Donald Trump's campaign communications director Tim Murtaugh said that campaign was "proceeding as normal". But the campaign hasn't announced another rally to follow the president's last one in North Carolina on March 2. Up until now, it had been keeping a steady pace of rallies this year.

Anthony Fauci, the National Institutes of Health's leading infectious disease expert, has stopped short of calling on candidates to put a pause on political rallies. But he also isn't faulting anyone who chooses to do so, saying that what "they're probably acting on is what they would consider for their citizens: an abundance of caution".