The U.S. Postmaster General announced the Postal Service will withhold absentee ballots in any state that does not share its voter data.
The move follows a new rule that requires states to disclose their mail voting lists.
If a state refuses, ballot delivery could be delayed or halted during an election.
The Postal Service states the requirement is essential to keep mail processing smooth.
Officials note the rule would give the agency the information it needs to sort and deliver ballots on time.
Critics warn the action could affect voters in tight races.