Voters in the US state of Wisconsin have elected a Democrat-aligned county judge to the state Supreme Court, defying a multimillion-dollar spend by Elon Musk.

The win for Susan Crawford on Tuesday (local time), which maintains the court’s 4-3 liberal majority, is a setback for US President Donald Trump and his billionaire ally Musk, who had backed her conservative rival Brad Schimel.

The election – less than three months after Trump returned to the White House – was widely seen as an early referendum on his presidency. He had weighed into the campaign repeatedly, urging voters to turn out and support Schimel.

The campaign easily became the most expensive judicial contest in US history with more than $US90 million ($A143 million) spent by the candidates, the state parties and outside groups, according to a tally from New York University’s Brennan Centre.

Crawford, a county judge, defeated Schimel, who is a former Republican state attorney-general and also a county judge.

With the balance of the court at stake, Musk and political groups tied to him spent more than $US21 million ($A33 million) to support Schimel.

Musk, whose so-called Department of Government Efficiency is overseeing Trump’s unprecedented cost-cutting campaign at the federal government, became a central figure in the race. He held a rally on Sunday night (local time) where his main super PAC, or political action committee, handed out $1 million cheques to two voters.

Wisconsin’s top court is likely to issue critical rulings on voting rights and election rules ahead of the 2026 midterms and the 2028 presidential race, when the state is expected to remain a central battleground. Trump won Wisconsin in November by less than a percentage point – the closest margin of any state.

The court is also poised to decide whether abortion rights should remain legal statewide and could revisit a Republican-backed law that stripped most public employee unions of collective bargaining rights.

Heather Williams, president of the Democratic Legislative Campaign Committee, celebrated Crawford’s win in a statement.

“Despite Republicans’ best efforts to buy this seat, Wisconsin voters showed up for their values and future,” she said.

“While Trump dismantles programs that taxpayers have earned, support, and are counting on, voters across the country are turning to state Democrats who are delivering on promises to lower costs and expand opportunities.”

-with AAP