The legal ruling that established the principle of "one person, one vote" was the 1964 Supreme Court case Reynolds v. Sims. This landmark decision mandated that state legislative districts must be roughly equal in population, thus ensuring that each person's vote carried equal weight. The ruling highlighted the need for fair representation in government and addressed the issue of gerrymandering, where district boundaries are manipulated to favor one party over another. As a consequence, it significantly advanced voting rights and empowered voters, particularly in urban areas that had previously been underrepresented in legislative processes.