There already exists a penalty for voter suppresion-the 14th amendment, section 2.
It says that when citizens right to vote is abridged then that state's congressional representation shall be reduced proportionately. I remember that that threat contributed to the passing of of "The Voting Rights Act".
Also a comment on the Supreme Court's verdict on the Voting Rights Act. They said that the Voting Rights Act has been working wonderfully. "Now stop it!"