Thompson, Mississippi's only congressional Democrat, sent the request Friday to U.S. Attorney General Eric Holder. Because of past discrimination, the Justice Department or a federal court must clear changes to Mississippi electoral procedures.
Mississippi voters approved photo identification before voting in a November 2011 referendum. The law must still win clearance, and federal officials want state Attorney General Jim Hood to provide information showing it won't have discriminatory effects.
Supporters say voter ID will prevent fraud. Opponents say it would make it harder for the poor and elderly to vote, doing outsized harm to black voters.
Even if the law wins approval, state officials say it won't be used for the Nov. 6 presidential election.