The Supreme Court on Monday turned down a last-ditch effort by the Trump administration to require California to separate mail ballots received from thousands of people it claims are not US citizens, leaving the state's count undisturbed in the final days of the midterm campaign.

The court issued a one-line, unsigned order denying the Justice Department's request for an injunction that would have required election officials to set aside ballots from 20,000 alleged noncitizens. The Justice Department had gone to the justices earlier in the day, after a divided federal appeals court refused to order California election workers to segregate mail ballots cast by people on a government list of "likely" noncitizens.

Voting in California has been under way since early October, and more than 6 million mail ballots have been returned to election offices across the state. There are more than 23 million registered voters in California.

How the case reached the justices

The administration told state officials and Los Angeles County last month that there were more than 20,000 suspected noncitizens on the voter rolls, and ordered that ballots received from those people be set aside and not tabulated until their citizenship could be confirmed. It then sent letters to officials in all of California's 58 counties with lists of alleged noncitizens and demanded that their ballots be put to one side.

Neither the district court nor the 9th US Circuit Court of Appeals agreed. In a Friday decision, two judges wrote that "there is a material risk that granting the United States its requested relief would result in disenfranchisement and could jeopardize the County's entire election".

The opinion also cited the National Voter Registration Act, which bars states from systematically removing ineligible voters within 90 days of a federal election.

A smaller number in practice

The actual number of ballots in dispute is likely much smaller than 20,000. County election officials have found that many of the people on the Justice Department's list did not have active voter registrations and were not mailed ballots.

Over the weekend, data scientists at the University of California, Los Angeles, Voting Rights Project were granted access to the list through a court order in another lawsuit. They found that 42 percent of the names on it do not appear on California's current voter roll.

California's attorney general, Rob Bonta, welcomed the ruling. "This latest decision should put an end to the Administration's attempt to disrupt California's elections through emergency court orders," he said in a statement. "We will remain vigilant and keep fighting back against federal efforts to undermine our democracy."

A wider pressure campaign

The Supreme Court's order is the latest turn in a broader federal push. In recent weeks the Justice Department sent subpoenas to election officials in California and reminded them that they could be criminally prosecuted for allowing ineligible voters to cast ballots. It has also brought criminal charges against more than 200 people who prosecutors allege voted illegally in past elections.

The same pressure has reached other states. Federal prosecutors sent a letter to Washington's secretary of state, Steve Hobbs, demanding he set aside mail ballots from more than 400 people they claimed were not US citizens. Hobbs said the government's list contained "many" people who actually are US citizens.

The federal government has appealed the earlier ruling, and the order at the centre of the dispute remains in place while it is reviewed by all active judges on the 9th Circuit.

What it means for the count

The practical effect of Monday's order is that California's counties can continue processing and counting mail ballots under their existing procedures while the legal fight continues. With voting already under way and millions of ballots in hand, any intervention at this stage would have been difficult to administer, which is part of why the lower courts refused.

Politically, the decision removes one immediate source of uncertainty from a state that is central to Democratic hopes of retaking the House. It does not end the dispute: the government's appeal is still live, and the underlying question — how states verify citizenship and when ballots can be challenged — remains unresolved.

The wider legal argument is unlikely to be settled soon. The government has not withdrawn its claim that thousands of ballots were cast by people who are not citizens, and the counties have not finished verifying the lists they were sent. Courts have so far preferred to let the count proceed while those questions are litigated, on the principle that removing ballots after they are cast is harder to undo than counting them. For election officials, the ruling means the work continues under scrutiny. County registrars have been told to keep the disputed names flagged so that any later order can be applied without recounting the whole ballot, and state lawyers are preparing for the possibility that the full appeals court reverses course. That preparation is the quiet consequence of Monday's order: the count goes on, but the file stays open. For voters who returned ballots early, it means their votes will be counted unless a separate challenge succeeds — a point state officials have stressed repeatedly as federal courts have weighed ballot disputes and orders against intimidating election workers. Nationally, voting day itself is now the next test of how the system holds up.