Under the Voting Rights Act of 1964, certain states are requried to get federal approval before they can change their voting laws. South Carolina is one of the states affected.
The Supreme Court to review Voting Rights Act
"The justices said they would decide whether Congress exceeded its
authority in 2006 when it reauthorized a requirement that states and
localities with a history of discrimination, most of them in the South,
receive federal approval before making any changes to their voting laws."