Article 15 grants "The right of citizens of the United States to vote shall not be denied or abridged by the United States or any State on account of race, color, or previous condition of servitude."
Source:
National Archives, http://www.archives.gov/global-pages/larger-image.html?i=/historical-docs/doc-content/images/15th-amend-voting-rights-l.jpg&c=/historical-docs/doc-content/images/15th-amend-voting-rights.caption.html
