The new coronavirus hampered efforts by voters in the United States to get to the polls in some states on Tuesday as the global pandemic left the Democratic presidential primary consumed with uncertainty.

Leaders in Ohio called off their primary just hours before polls were set to open, citing the need to combat the virus. Voting moved forward in Florida, Illinois and Arizona, but challenges emerged as some poll workers did not show up and those who did, tried to create distance among voters to comply with new health guidelines limiting large gatherings.

Also on Tuesday, Maryland became the fifth state to announce that it would postpone its primary election. The vote, originally scheduled for late April, will now take place in at the end of June.

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In Florida, the Palm Beach County elections department said many workers failed to show up in at least five locations. The county had 800 volunteers back out as of Monday, with just 100 new volunteers offering to take their place.

In Illinois, Cook County Clerk Karen A Yarbrough encouraged poll workers to mark space on the floor at polling sites to keep voters a safe distance apart and avoid spreading the virus.

The Clerk's Office encourages #ElectionDay pollworkers to #MarkTheFloor in 6 foot increments with their blue painter's tape to remind voters to practice social distancing.



It is our job to ensure the safety of those around us while we carry out our civic duty today. pic.twitter.com/QXmBCskCQH - Karen A. Yarbrough (@cookcountyclerk) March 17, 2020

Yarbrough said shortly after the polls opened, however, she had heard of no problems at the county's polling sites and expected "things to go well" on Tuesday.

On a call with reporters later in the morning, however, election officials in Chicago said turnout in that city was extremely low - about a fourth of what they would normally expect.

Not since New York City postponed its mayoral primary on the day of the September 11, 2001, attacks has an election been disrupted in such a high-profile, far-reaching way. That was especially true in Ohio, where Governor Mike DeWine initially asked a court to delay the vote. When a judge refused to do so, the state's health director declared a health emergency that prevented the polls from opening.

The decision was a reminder that the most elemental act of American democracy - voting - will be severely tested Tuesday as several states hold presidential primaries while also confronting the effect of a global pandemic. The contests are playing out as the virus' effect is becoming more tangible with schools closing across the country, workers staying home and restaurants and bars shuttering.

In the run-up to the election, campaigns spent Monday sifting through data and talking to contacts on the ground to assess the effect of the coronavirus on turnout. Former Vice President Joe Biden is moving closer to securing the Democratic presidential nomination but could face a setback if the older voters who tend to support him do not show up. Vermont Senator Bernie Sanders, meanwhile, cannot afford to lose support from young voters who have been his most loyal supporters.

The tumult has left the campaign in a state of suspended animation. In-person rallies have been replaced with sometimes-awkward virtual events.

Sanders, the last Democrat standing between Biden and the nomination, is not planning to drop out. His campaign looked to have nowhere to go after a big loss last week in Michigan, and another blow landed on Monday night when Biden was declared the winner of the primary in Washington state, giving him victories in five out of six states that voted March 10. Yet Sanders's top advisers see no downside to staying in the race as they assess how the coming days and weeks unfold.

Democratic presidential candidates Bernie Sanders and Joe Biden at a debate in Washington, DC [Kevin Lamarque/Reuters]

"I don't have to tell anybody that we are living in a very unprecedented and strange moment in the history of our country," Sanders said during a virtual rally on Monday, urging supporters that it may be time to "rethink our value system, rethink many of the systems we operate under".

Still, Sanders faces an increasingly tough path to the nomination. About half of the delegates in the Democratic primary have already been awarded and, if Biden has another big night on Tuesday, he will pad an already large and perhaps insurmountable lead. Sanders trails Biden by more than 150 delegates nationally, meaning he would need to win more than 57 percent of those yet to be allocated to clinch the Democratic nomination.

Biden's campaign is trying not to look presumptuous about its prospects at this sensitive moment. Still, the former vice president is making moves to rally more voters to his campaign, including his announcement during the debate that he would choose a woman as a running mate.

Biden appeared to keep his focus on Monday on winning the nomination, as he encouraged voters in a telephone town hall to participate in Tuesday primaries but to do so safely.

Joining him was former Surgeon General Dr Vivek Murthy, who served during President Barack Obama's second term. Murthy encouraged voters at high risk of contracting coronavirus to vote by mail or use curbside voting, if available, but he also explained precautions elections officials are planning in the Tuesday primary states.

The coming weeks will present additional uncertainties. After Tuesday, the campaign had been set to shift to Georgia next week, but officials there have already postponed their Democratic primary until May 19. That means voting is not scheduled again anywhere until March 29 in Puerto Rico - and island officials are also seeking a delay.

The first week in April, meanwhile, would have featured Louisiana, but its decision to delay the primary until June 20 leaves only primaries in far-flung Alaska and Hawaii and caucuses in Wyoming through April 4.

Voting rights groups have advocated for upcoming elections to be postponed, or for states holding them as scheduled to adopt more lenient vote-by-mail and absentee ballot rules so that people do not have to choose between showing up at a polling place and putting their health at risk.

But Mustafa Tameez, a Democratic strategist with ties to many of the party's top donors, noted that Americans voted during World War I and World War II.

"There should be no circumstance in which we say, because of a crisis - regardless of the crisis - that we stop our electoral government," Tameez said.