Wisconsin voters have elected a Democratic-backed judge to serve on the state supreme court, according to projections, following the most expensive judicial election in US history.

Susan Crawford is on course to beat conservative rival Brad Schimel, which would keep intact the 4-3 liberal dominance of the Midwestern state's highest court.

President Donald Trump's billionaire adviser Elon Musk was a prominent fundraiser in the campaign, and was the subject of attack ads aired by Crawford's supporters. More than $100m (?77m) was spent by the candidates and their allies, including $20m from Musk.

The result is expected to have far-reaching implications, potentially even affecting the balance of power in the US Congress.

With about three-quarters of ballots tallied, Crawford had won about 55% of the vote, and Schimel had around 45%, according to the BBC's US partner CBS.

The Dane county judge was formerly a private lawyer for Planned Parenthood and she backed abortion rights during her campaign.

Tuesday's result was a setback for Trump in a crucial presidential swing state that he won by less than a percentage point last November.

However, he took consolation from fellow Republicans managing to hold on to two congressional seats in Florida elections on Tuesday.

Earlier in the day on his Truth Social platform, Trump reiterated his support for "patriot" Schimel, a Waukesha County judge and former Republican attorney general. The president had warned Crawford would be "a DISASTER for Wisconsin".

The judicial contest was seen as a test of Musk's powerbroking status. The SpaceX and Tesla boss travelled to the state to give out millions of dollars to voters who pledged to support conservative causes.

In the city of Milwaukee, which leans Democratic, officials reported a shortage of ballots on Tuesday "due to unprecedented and historic voter turnout", the city's election commission said in a statement.

Wisconsin separately voted on Tuesday to enshrine into the state constitution a law requiring voters to show ID to cast their ballots.

Voters were already required to show ID, but adding it to the state constitution made it harder to change in the future. Crawford had opposed the voter ID constitutional amendment.

Wisconsin's supreme court is expected to play a key role in several upcoming cases, including laws around abortion and congressional redistricting ahead of Midterm elections in 2026 and the next presidential election, in 2028.

Getty Images Schimel wears a black shirt and speaks into a microphone with a US flag behind him Getty Images

Schimel had received support from top Republican party officials

At an NBA game in Milwaukee on Tuesday, several voters spoke to the BBC about their concerns.

Milwaukee Bucks fan Mike McClain said he was motivated by a dislike for Musk, who he referred to as "the real president".

"I don't know how a billionaire, almost a trillionaire, can decide what's going on," he said. "You can't even relate with common people."

Crawford also benefited from large donations by billionaire donors, including financier George Soros, Illinois Governor JB Pritzker and LinkedIn co-founder Reid Hoffman. But Musk outspent them all.

A Schimel supporter who did want to give his name said he was supporting the conservative out of loyalty to Trump.

"We got to take it back home here and reinforce everything that Donald Trump has done," he said.

Much of the liberal campaign focused on the role played by Musk in the Department of Government Efficiency, a cost-cutting task force that has moved to fire thousands of government workers and slash the federal payroll.

During a rally on Sunday, Musk distributed two $1m cheques to voters at a rally who signed a petition of his against "activist judges".

Others who signed it received $100 from Musk.

On Tuesday, Musk's political action committee added that it would pay $50 to anyone who snapped a picture of a Wisconsin resident standing outside a polling site and holding a photo of Schimel.

Musk donated more than a quarter of a billion dollars to help elect Trump. He held similar $1m giveaways to boost the Republican president's campaign last year.

Wisconsin's supreme court is expected to play a key role in determining the shape of congressional districts if Democrats seek to challenge current district maps as they are widely expected to do.

Republicans currently hold six of the state's eight seats in the US House of Representatives.

At his rally on Sunday, Musk alluded to the looming fight over congressional districts, saying the judicial race was ultimately about control of the US House of Representatives, where Republicans currently hold a narrow majority.

That slender margin was shored up on Tuesday in special congressional elections in Trump's political heartland of Florida.

Republican candidates Jimmy Patronis and Randy Fine held on to those ruby-red seats in races that were seen as a barometer of the political landscape ahead of next year's Midterm elections. (BBC)

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