Ohio has called off its presidential primary just hours before polls were set to open there and in three other US states.
Ohio's governor Mike DeWine said the 11th-hour decision was necessary to prevent further fuelling the coronavirus pandemic that has paralysed the nation.
Ohio Health Director Amy Acton declared a health emergency that would prevent the polls from opening out of fear of exposing voters and volunteer poll workers to the virus.
Arizona, Florida and Illinois are proceeding with their presidential primaries.
DeWine failed to get a judge to halt the primary Monday evening, even though the governor contended the election results wouldn't be viewed as legitimate in light of the pandemic.
"To conduct an election tomorrow would would force poll workers and voters to place themselves at a unacceptable health risk of contracting coronavirus," he said.
It wasn't clear what would happen, but DeWine said officials were considering how to give voters an opportunity to cast their ballots.
Officials in Arizona, Florida and Illinois felt they had done enough to ensure the safety of voters, even as concerns mounted that there will not be enough poll workers in some areas and voters will be confused after polling places in nursing homes were moved to other locations.
Elsewhere, Georgia, Kentucky and Louisiana have postponed their scheduled primaries.
Turnout at polling places is already expected to be light Tuesday as only the Democrats have a contested presidential primary, and that is down to two contenders: Joe Biden and Bernie Sanders.
The polling states have been taking steps to limit voter and poll worker exposure to the coronavirus.
In Florida and Arizona, the states moved polling places located in nursing homes and assisted living facilities to avoid exposing the residents to outsiders.
Meanwhile, Joe Biden has been declared the winner of last week's Democratic presidential primary in Washington state, giving him victories in five out of six states that voted on March 10.
After nearly a week of counting votes, the former vice president held onto a small lead over Vermont Senator Bernie Sanders that turned out to be insurmountable.
Washington was a state that Sanders had been hoping to win.
In 2016, he won more than two-thirds of the delegates from the Washington caucuses over Hillary Clinton.
Of the state's 89 pledged delegates, only 31 are allocated based on the statewide result.
The remaining 58 are determined based on the results of the state's 10 congressional districts, and those results might not be calculated until the election is certified by the secretary of state's office, which could be as late as March 27.
Biden won four other states last Tuesday: Missouri, Mississippi, Michigan and Idaho. Sanders won North Dakota.