The US Supreme Court has rejected a last-minute request by the Trump administration asking it to intervene in California's midterm election vote count.
The justice department wanted 20,000 ballots to be set aside that it alleged may have been issued to non-US citizens in the state.
Two lower courts had already rejected the request. State election officials had questioned the federal government's claims - saying the process used to identify alleged non-citizens was inaccurate.
The Monday ruling comes on the eve of the midterms, which will determine which political party controls the US Congress for the last two years of President Donald Trump's second term.
The Supreme Court announced its decision in a one-line order, saying it was denying the justice department's request for an injunction.
"The application for injunction pending appeal presented to Justice Kagan and by her referred to the Court is denied," it reads.
Its means rulings made in lower courts will stand and essentially ends the Trump administration's bid to disqualify the ballots.
California is the most populous state in America with some 23 million people on its voting register. As of Sunday night, more than six million mail-in ballots had already been received, according to the California secretary of state's office.
The justice department sent letters to the California secretary of state and Los Angeles County Registrar last month, demanding they set aside the ballots of 20,000 registered voters whom the Trump administration "believe" are "likely" non-citizens, according to court documents.
About a week later, armed officials from the Department of Homeland Security delivered letters to county officials and employees across California threatening criminal charges if they didn't sequester those ballots.
In a 22-page brief reviewed by the BBC, the justice department maintained that ballots cast by non-citizens could "debase or dilute valid votes".
On Friday, a federal appeals court declined the government's request to segregate the ballots.
California election officials have questioned the accuracy of the government's claims, and have said the list of names the government has provided is riddled with inaccuracies.
They have expressed concern that eligible voters could be disenfranchised and said segregating ballots whilst voting is underway is unfeasible.
The attempts are a part of a years-long campaign by Trump and his administration to fight alleged voter fraud that they believe is widespread, despite research showing that it is extremely rare.
President Trump has repeatedly claimed that Democrats steal elections by allowing immigrants to vote illegally, and he blames much of it on the so-called "mail-in" or postal voting that is permitted in many Democratic states.
Last week in a scathing ruling, district judge David Carter said the government's move in California was indicative of an "ongoing pattern of attempted election interference" on the part of the Trump administration.
The judge added that "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".
With additional reporting by the BBC's David Willis
