The actual number of ballots in dispute is likely much smaller than 20,000, since county election officials have found that many of the people on DOJ's list did not have active voter registrations and were not mailed ballots.

The Justice Department's appeal to the Supreme Court comes as it has been escalating its feud with California election officials.

In recent weeks, DOJ officials have warned local election officials they could be criminally prosecuted if they do not cooperate with federal demands to segregate ballots. Last week, the Justice Department sent grand jury subpoenas to employees at the Los Angeles County Registrar-Recorder's office.