The Fifteenth Amendment assigns Congress — not the Supreme Court — the responsibility of ensuring that no citizens are denied their right to vote. Section 5 has faced at least two constitutional challenges in the past, and survived both of them.
Read the full article by Raul A. Reyes, Attorney; USA Today Columnist at http://www.huffingtonpost.com/raul-a-reyes/voting-rights-act_b_2194360.html
The Voting Rights Act of 1965: http://www.justice.gov/crt/about/vot/intro/intro_b.php