Other laws they want assessed are those that don't allow people to present certain forms of identification, such as a Social Security card, those that require proof of citizenship, and those that preclude same-day voter registration.

Disputes over how to properly identify legal voters have risen over the last year, as some states have sought to require these sorts of changes in order to help prevent illegal residents from voting. But Democrats in particular have opposed these changes, and many have said they would have the effect of making it harder for blacks to vote.

The letter from Conyers and Nadler sites a study from the Brennan Center for Justice that said these changes would hurt minority voters. "The report concluded that African-American and Hispanic voters were more likely to take advantage of early voting opportunities and register to vote through the types of voter registration drives now curtailed or eliminated by the new laws," they wrote.

"In view of the gravity of this situation, we urge you to schedule hearings soon to address an issue so critical to our democracy," they concluded.