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.