Bernie Sanders says that Wisconsin's in-person primary election voting is "dangerous" and "disregards the public health experts" as people across the state crowd polls despite warnings against large gatherings and a statewide stay-at-home measure during the coronavirus pandemic.

The state's Republican leadership and conservative majority on the state Supreme Court blocked last-minute efforts to postpone the election, as many states have done, with millions of Americans being directed to shelter in place and avoid crowds.

Senator Sanders will not be hosting any traditional campaign events on Tuesday.

He said it's "outrageous" that state officials "are willing to risk the health and safety of many thousands of Wisconsin voters ... for their own political gain".

In-person voting could "very well prove deadly," he said.

Long and crowded lines of people — many not wearing masks or bound by "social distancing" measures to keep people at least six feet apart — have formed across Milwaukee, the state's largest city, which has just five polling sites, down from 120.

Some voting sites have reported waits of up to five hours.

Governor Tony Evers's attempt to postpone the election through an executive order was blocked by the state's Supreme Court on Monday, and the US Supreme Court also blocked a deadline extension for mail-in or absentee ballots.

The Wisconsin Election Commission predicts hundreds of thousands of people at the polls on Tuesday.

Meagan Wolfe, the state's chief elections official, urged voters to be "careful and patient" at the polls, while local election officials have stocked up poll sites with "sanitation supplies and have been trained on social distancing procedures and other guidance developed with a public health official."

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Governor Evers insisted on Monday that his attempts to delay the election were not motivated by politics but in protecting residents.

He said: "The people of Wisconsin, the majority of them, don't spend all their waking hours thinking about are Republicans or Democrats getting the upper hand here.

"They're saying they're scared. They're scared of going to the polls. They're scared for their future. At the end of the day, someone has to stand up for those folks."

Joe Biden, who is leading against Senator Sanders for the Democratic nomination to face Donald Trump in the general election, called for a "virtual" Democratic National Convention in August, which also will be held in Wisconsin.

But he told viewers on a virtual town hall this week that "tens of thousands" of people at a convention is a different scenario than in-person voting, with voters adhering to "accurate spacing" and going into polls "one at at time" with machines "being wiped down".

His campaign also has assured that voters could safely participate in 17 March primaries in three states, where similarly crowded lines formed in the midst of the pandemic; Ohio postponed its primary election scheduled for that day.

Dissenting justices on the US Supreme Court said moving forward with today's election while the US is in the middle of a crucial quarantine effort to prevent the spread of the virus will ultimately result in the "massive disenfranchisement" of thousands of voters who fear showing up at the polls.

Liberal justice Ruth Bader Ginsburg said: "The question here is whether tens of thousands of Wisconsin citizens can vote safely in the midst of a pandemic."

The court's decision "forces voters to choose between endangering their safety by showing up in person or losing their right to vote" on Tuesday, she wrote.

Many of Senator Sanders' supporters have urged him to consider boycotting the election or denounce its results as illegitimate, arguing that pressure to hold a traditional election in the middle of a public health crisis will prevent many voters from participating, and skewing the results.

Voters also fear that poor turnout could dramatically change outcomes in the dozens of crucial local races that also appear on ballots across the state.