The US Department of Justice sued California on Thursday, seeking a court order to prevent the state from tabulating certain ballots cast in the midterm elections.

The suit came two days after the department made a specious demand to the state. On Tuesday, the department sent election officials in Los Angeles county a thumb drive with a handwritten password purporting to contain the names of noncitizens registered to vote. The department asked officials to confirm the citizenship of anyone on the list and set aside ballots from those whose status could not be confirmed. It gave the state 48 hours – an extremely narrow window – to take action. It did not provide details on how it came up with the names on the list. In its lawsuit, filed in federal court in the central district of California, the justice department said it had provided the state with more than 20,000 names of likely noncitizens.

The Los Angeles county clerk and recorder, Dean Logan, said his office would not review the list, in part because it didn’t consider the USB drive secure.

In a statement on Wednesday, the California secretary of state, Shirley Weber, a Democrat, said the justice department had not provided the list of voters to her office. “The [justice department’s] relentless efforts to unlawfully gain access to states’ protected voter data and purge voter rolls serve only to cause chaos and fuel public distrust in our elections,” Weber said.

Justice department lawyers are asking the court to order the state not to count votes from those it had flagged.

“This Court should enjoin California from presently tabulating vote-by-mail ballots from likely noncitizens in the upcoming federal election until the voter’s citizenship status can be determined,” the lawsuit said. “This Court should order California (1) to set aside vote-by-mail ballots from persons identified by the Federal Government likely to be noncitizens and (2) to independently – or in collaboration with the Federal Government – verify their citizenship status (beyond relying on unverified statements of citizenship necessary to registering) before tabulating their vote-by-mail ballot.”

The request to block tabulation comes amid concerns that Donald Trump and his allies will interfere in the counting of votes in the midterm elections, in which Republicans are expected to lose seats in Congress.

The Trump administration’s methodology for identifying noncitizens has come under scrutiny. Earlier this year, the Department of Homeland Security (DHS) said there could be as many as 278,000 noncitizens on the rolls in four states, including more than 190,000 in California and about 16,000 in Nevada.

Officials at the DHS refused to share their methodology with Nevada election officials and later turned over just 185 names of people it had a strong belief were noncitizens. When Nevada officials investigated, they found every one of those people was a citizen.

Richard Hasen, an election law scholar at UCLA, described the justice department’s efforts in California as “a nice publicity stunt that gets attention for [the justice department] and reinforces the notion among the president’s most ardent supporters that California is not doing enough to combat voter fraud”.

“The Department of Justice has no standing to tell states and localities to go in and remove voters from voting rolls in the midst of an election. In fact, the National Voter Registration Act bars systematic purges of ineligible voters in the 90-day period before the election,” he wrote in a blogpost. “The purpose of this provision, unsurprisingly, is to stop false positives, that is the removal of eligible voters who are mistakenly flagged as noncitizens. A rushed process would almost certainly lead to disenfranchisement.”