The landmark case that solidified the principle of "one person, one vote" in U.S. elections was Westberry v. Sanders decided in 1964. This Supreme Court ruling mandated that congressional districts must have roughly equal populations, ensuring equal representation in the U.S. House of Representatives. The decision emphasized the importance of electoral fairness and the idea that every citizen's vote should carry equal weight. It paved the way for subsequent redistricting reforms and increased public awareness of voter rights, significantly shaping the landscape of American democracy and ensuring greater equity in the electoral process.