In Guinn and Beal v. United States, 1914, the U.S. Supreme Court ruled that Oklahoma's "grandfather clause," which exempted citizens from literacy tests for voting if their grandfathers had been eligible to vote before 1866 (effectively excluding African Americans), was unconstitutional as it violated the Fifteenth Amendment of equal voting rights regardless of race or color. The case involved two African American men, John Guinn and Percival Beal who were denied voter registration in Oklahoma due to this clause. They sued on grounds that it infringed upon their constitutional right to vote without racial discrimination. The court agreed with them unanimously, marking a significant victory against racially discriminatory voting practices at the time.

In the dissenting opinion for Guinn and Beal v. United States, Justice Holmes argued that Oklahoma's grandfather clause was not inherently discriminatory or designed to disenfranchise African American voters. He contended that it was a legitimate attempt by the state to ensure voter literacy and political knowledge among its electorate, which is within its rights under the Constitution. Furthermore, he believed that if any discrimination did occur as a result of this law, it would be incidental rather than intentional - an unfortunate side effect of a well-intentioned policy rather than evidence of systemic racism or bias. Therefore, in his view, striking down this law based on potential misuse or unintended consequences could set a dangerous precedent for future cases involving states' rights and voting laws.