Nixon v. Herndon (Texas) struck down a 1923 Texas law that prohibited blacks from voting in the Democratic Party primary

In 1902 the Texas legislature passed a requirement for a poll tax which suppressed voting by black and Mexican Americans. As voter participation by these groups declined, the Democratic Party became more dominant.[2]

Dr. Lawrence A. Nixon, a black physician in El Paso, Texas, and member of the Democratic Party, sought to vote in the Democratic Party primary of 1924 in El Paso.[3] The defendants were magistrates in charge of elections who prevented him from doing so on the basis of the 1923 Statute of Texas which provided that “in no event shall a negro be eligible to participate in a Democratic Party primary election held in the State of Texas”. Nixon sought an injunction against the statute in the federal district court. The district court dismissed the suit, and Nixon appealed to the United States Supreme Court.

Source: wiki