The Fifteenth Amendment

"Section 1.

The right of citizens of the United States to vote shall not be denied or abridged by the United States or by any State on account of race, color, or previous condition of servitude. 

Section 2.

The Congress shall have power to enforce this article by appropriate legislation."

This amendment gave the ex slaves the right to vote in 1870 when it was ratified, but many were still denied voting rights through 1964-1965.  

This video is showing the connection between the 13th, 14th and 15th Amendments in the Constitution.  

This image shows how different Congress men tried to use the fifteenth amendment to gain votes for an election.