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.

Last week, the Justice Department sued the state and later warned that local election workers who knowingly process or tabulate ballots submitted by noncitizen voters are at risk of potential prosecution.