The Trump administration asked the US Supreme Court on Monday to force California to set aside the mail-in ballots of more than 20,000 people it claims may not be American citizens.
The move, one day ahead of the crucial midterm elections, comes after lower courts last week rejected the demand by the Department of Justice (DOJ), with one judge calling it brazen pre-election "interference."
Solicitor General D John Sauer, in an emergency filing with the Supreme Court, said that "once California commingles the challenged ballots with lawful ones, there is no way to get the challenged ballots back."
"The only way to preserve the district court's ability to grant relief to the United States after a full merits proceeding, therefore, is to segregate the challenged ballots now," Sauer said.
The California ballot case appears to represent another attempt by the administration to cast doubt on the results of Tuesday's vote, in which President Donald Trump's Republican Party is at risk of losing control of Congress.
Trump -- who has never acknowledged his own election defeat by Democrat Joe Biden in 2020 -- has been claiming for months that there will be fraud in this year's midterms.
District Judge David Carter last week refused to order the seizure of the ballots and sided with the government of Democratic-led California.
"Never before in American history has the federal government, on the eve of an election, sought to coerce a state and local government to abandon their own Constitutional elections procedures and bend to the will of the Executive Branch," Carter said. "The DOJ's actions are indicative of a broad, ongoing pattern of attempted election interference by federal officials."
The federal government in recent months had accused California of having 190,000 illegal voters on its books. It recently revised the figure to 20,000 and filed an emergency lawsuit -- after early voting had begun -- to stop the state from sending ballots to them.
A similar controversy arose in neighboring Nevada. The administration said initially it had identified 16,000 non-citizen voters there, but after verification procedures that number dropped to 185 -- and then to zero.
Studies repeatedly show that while rare cases of voter fraud exist, such incidents are miniscule and do not influence election results.
In another closely-watched election case, a federal judge in Alaska over the weekend tossed out a bid by the Trump administration to hold up US Senate election results in the state should the Democratic candidate win.
The DOJ had asked the court to block certification of the results if incumbent Republican Dan Sullivan loses on Tuesday because a second candidate also named Dan Sullivan would have siphoned off votes.
Mary Peltola, the Democratic candidate for Senate in Alaska, has denied recruiting the second Sullivan into the race but The New York Times reported that members of her campaign staff had been involved.