The supreme court on Monday, 2 November, ruled against the Trump administration’s effort to require California election officials to separate and withhold from counting ballots cast by people the justice department alleges are non-citizens. Several lower courts had already sided with state officials, who argued that the federal government was attempting to interfere in California’s elections.

The dispute began after the justice department claimed that 20,000 non-citizens were included on California’s voter rolls. In mid-October, the department supplied election officials in Los Angeles and Sacramento with a USB drive containing names. It allowed them 48 hours to examine the list and threatened legal action unless ballots associated with those individuals were set aside pending verification of their citizenship. Officials declined to comply, triggering court proceedings.

California officials and voting rights groups challenged the allegations and questioned how the department had assembled its information. They accused the administration of trying to undermine confidence in the midterm elections. The department declined to explain its methodology, while election officials’ analysis identified numerous indications that the list was inaccurate. Los Angeles county clerk and recorder Dean Logan said the department had supplied neither supporting evidence nor an explanation of the sources and methods behind its assertions of voter ineligibility.

As the legal confrontation intensified, the administration threatened to prosecute election officials who processed ballots from ineligible non-citizens. Experts said such prosecutions would have no legal basis. The administration also launched a criminal investigation into allegations of non-citizen voting in California and began issuing subpoenas to election officials.

On 28 October, US district judge David Carter rejected the administration’s request to separate the ballots. Carter, a Bill Clinton nominee, described the federal intervention as an extraordinary attempt to pressure state and local authorities into abandoning their constitutional election procedures shortly before an election.

The California case forms part of a broader federal effort to expand its role in election oversight and advance allegations of voting fraud by non-citizens. The justice department has sought similar ballot segregation in Washington state. It has also brought criminal charges against 28 green card holders in California, accusing them of registering fraudulently and casting unlawful ballots.

Trump has separately made vague promises of a $1m reward for information about voter fraud that results in an arrest or conviction. Despite the administration’s focus on the issue, data and experts have shown that voting by ineligible non-citizens is extremely rare.