Final answer:
The U.S. Supreme Court in Smith v. Allwright (1944) declared racial exclusion in primary elections unconstitutional, interpreting the Fifteenth Amendment as applicable to state-regulated primary elections and thereby ensuring voting rights for all citizens regardless of race.
Step-by-step explanation:
In the landmark 1944 Supreme Court case Smith v. Allwright, the court ruled that the practice of denying African Americans the right to vote in primary elections was a violation of the Fifteenth Amendment. This legal challenge was brought forth when Lonnie Smith of Houston was denied the ability to vote in the Democratic primary due to a racial exclusion policy upheld by the Democratic Party of Texas. They claimed that because they were a private organization, such exclusion did not contravene the Amendment. However, the Supreme Court decided that since primary elections were state-regulated, they must comply with Constitutional mandates that prohibit racial discrimination. As a consequence, the decision meant that the right to vote in primary elections was guaranteed to citizens regardless of race, and it signified a major step forward in the fight against voter suppression and for civil rights.