Wisconsin will hold its in-person primary election on Tuesday after the state's Republican-dominated supreme court voted along party lines to overturn the Democratic governor's bid to delay it amid the coronavirus pandemic.

Earlier on Monday, governor Tony Evers had said he was planning to postpone the primary until 9 June to protect voters from the risk of spreading Covid-19, overriding the objections of Republican politicians in the state. He had previously opposed moving the primary himself.

However, the supreme court voted 4-2 that he did not have the legal right to postpone the vote on his own authority.

Many Democratic primaries have either been postponed – several until 2 June – or else switched to mail-in ballots, such as this month's contests in states including Alaska, Wyoming and Ohio, because of the pandemic.

Mr Evers himself had questioned whether he had the power to reschedule the election, but said the worsening situation, including an increase in Covid-19 deaths from 56 on Friday to 77 on Monday, made it clear there was no way to safely move forward. He insisted he was motivated by protecting public health, not politics.

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."

Republicans quickly took their case to the conservative-leaning Wisconsin Supreme Court, which ruled in their favour.

Dan Kelly, one of the conservative judges who is also on the ballot on Tuesday, recused himself from the case and then commented on Twitter that the election can be done safely and should be allowed to proceed.

On Thursday Donald Trump said he believed the Wisconsin primary should still go ahead.

A polling station in Waukesha, Wisconsin (AP)

Asked if he believed the governor was trying to delay the election for reasons of public safety, the president said: "I don't know. Why didn't he do it before? Excuse me. Why didn’t he do this two weeks ago?"

Joe Biden has a commanding lead over his rival Bernie Sanders in the polls, despite the democratic socialist having won the state when he was running against Hillary Clinton for the nomination in 2016.

The Associated Press contributed to this report