President Donald Trump's administration asked the US Supreme Court on Monday, November 2, to prevent California from counting mail-in ballots from more than 20,000 registered voters suspected by federal officials of being noncitizens.
The emergency request, filed one day before the midterm elections, escalates a legal dispute over voter eligibility and state authority to administer elections.
California officials argue that the federal government's allegations rely on unreliable data and could disenfranchise eligible voters.
The Justice Department wants California to separate the contested ballots and withhold them from counting until officials verify each voter's citizenship.
The list was compiled by the Department of Homeland Security and shared with California and Los Angeles County officials.
Federal authorities sued California on October 15, alleging that the state was obstructing enforcement of a 1996 law prohibiting noncitizens from voting in federal elections.
The administration also argues that California's actions violate the Constitution's Supremacy Clause.
California maintains that its existing election safeguards are sufficient and that the federal government's screening methods risk incorrectly identifying naturalized citizens as ineligible voters.
State officials say separating the disputed ballots would be extremely difficult because voting has been underway since early October.
US District Judge David Carter rejected the administration's request on October 28, questioning the reliability of federal voter data and warning against executive interference in ongoing elections.
The Ninth US Circuit Court of Appeals subsequently refused the request on Friday, emphasizing California's authority over election procedures.
The appeals court warned that blocking ballots could disenfranchise voters and jeopardize election administration.
California Attorney General Rob Bonta criticized the administration's intervention, while acting Deputy Attorney General Trent McCotter defended continued legal efforts to enforce federal election law.
The Supreme Court request comes as Republicans seek to retain congressional control in Tuesday's elections.
California holds 52 House seats, while Democrats need a net gain of four seats to secure a majority.
The administration has also pursued broader challenges involving voter rolls and mail-in voting, despite studies and state audits finding noncitizen voting to be rare.