The US Supreme Court on Monday turned down a request from President Donald Trump's administration to force California to cancel mail-in ballots cast by more than 20,000 people the government claimed might not be US citizens.

The court issued a brief, unsigned order without explanation, a day before the midterm elections.

According to Agence France-Presse, the federal government initially accused California of illegally placing 190,000 people on its voter rolls before lowering the figure to 20,000. After early voting had begun, it filed an emergency lawsuit to stop the state from sending ballots to those voters.

The Supreme Court's ruling followed rejections by lower courts last week and again on Monday. One judge called the Justice Department's move a flagrant intrusion into the electoral process. Federal Judge David Carter, who last week declined to order the ballots seized and sided with California's Democratic-led authorities, said the department's actions reflected "a pattern of attempts to interfere in elections." Judge James Boasberg described the lists of suspected non-citizens as "flawed."

In a similar case in neighbouring Nevada, the administration said it had initially identified 16,000 non-citizen voters, a figure that fell to 185 after verification, and then to zero.

In Alaska, a federal judge rejected a Justice Department request to block certification of the state's Senate race should incumbent Republican Senator Dan Sullivan lose. The department had argued that another candidate with the same name may have drawn away some of his votes.

Trump has for months raised the prospect of fraud in the midterms, with forecasts suggesting Democrats could win control of at least one chamber of Congress.