The administration has alleged California sent ballots to some 20,000 people who may not be U.S. citizens. It wants the state to set those ballots aside and not count them until citizenship can be verified.

A federal judge rejected on Wednesday a Trump administration request to stop California from processing the ballots of up to 20,000 people over allegations they are likely noncitizens, warning it would "wreak havoc" on the midterm elections.

The administration has insisted that immigrants voting is a major problem nationwide, when in fact it is quite rare, as even its own investigations have shown.

The letters are the latest escalation in a legal battle between California and President Donald Trump's Department of Justice, which has recently focused on the state in its efforts to root out noncitizen voting ahead of the midterm elections.