A federal judge ruled against the Justice Department’s use of voter data to cross-reference an immigration database.

A United States judge has ruled that the Justice Department unlawfully sought state voter rolls to cross-check against a federal immigration database and identify voters it considered ineligible.

US District Judge Sparkle Sooknanan’s decision on Friday marks another setback for President Donald Trump’s efforts to expand federal oversight of elections ahead of the midterms, as Republicans seek to retain control of Congress.

In a 78-page ruling siding with watchdog group Common Cause, Sooknanan said the administration had understated the risk of wrongly removing eligible citizens from voter rolls because of inaccuracies in the Systematic Alien Verification for Entitlements (SAVE) database.

“The challenged policy represents the federal government’s attempt to decide who gets to vote,” Sooknanan wrote, adding that Congress had left that authority to the states.

The Justice Department said it was reviewing the ruling.

Ming Cheung, a senior staff attorney with the American Civil Liberties Union’s Voting Rights Project, said the ruling made clear that the administration could not use the federal government to disrupt elections or put voters’ rights at risk.

“The Trump administration has repeatedly taken extreme measures to interfere with how elections are run, undermine confidence in the results, and create confusion,” Cheung said in a statement.

Last year, Trump directed the department to crack down on voting by ineligible people, despite research showing voter fraud is rare. The department subsequently demanded unredacted voter rolls from nearly all 50 states, including sensitive personal information.

Eighteen states, mostly Republican-led, complied. The department cross-checked their data against SAVE and flagged people identified as noncitizens for potential removal from voter rolls.

Sooknanan said the database could contain outdated citizenship records, potentially misidentifying naturalised US citizens.

The department has also sued 30 states and Washington, DC, over their refusal to provide voter data. Twenty-six cases have been dismissed.

In a separate case in June, Sooknanan blocked the expanded use of the SAVE database for voter roll checks, an order the US Supreme Court temporarily paused last month.

Federal law bars systematic voter roll purges in the 90 days before a federal election, which could limit the ruling’s practical effect before the November 3 vote.