The U.S. Supreme Court case that established the principle of "one person, one vote" is Baker v. Carr (1962). This landmark ruling addressed the issue of apportionment and unequal representation in legislative districts. The Court held that legislative districts must be approximately equal in population, ensuring that each citizen's vote carries equal weight. This decision significantly impacted the redistricting process, leading to fairer electoral representation and encouraging greater participation in the democratic process. It reflected the importance of electoral equity in upholding the foundational tenets of democracy.