The decision in King v. Chapman (1945) was key in ending the "White Primary" in the United States, that is to say, the local "rules" that the primary elections were only for white people.
The case originated because of the lawsuit made by Primus E. King, an African American and a registered voter who was denied his right to vote in the Democratic Party’s primary election of Columbus, Georgia because according to this state statutes, the primaries were only for Whites.
However, in Court, it was decided that the exclusion of black voters was unconstitutional under the Fourteenth, Fifteenth and Seventeenth Amendments. And therefore, the "White Primary" was prohibited in all states of America.