The landmark Supreme Court case that established the principle of "one person, one vote" was Baker v. Carr, decided in 1962. This case addressed the issue of legislative apportionment in Tennessee, where districts were unequal in population, undermining equal representation. The Supreme Court ruled that federal courts had the authority to intervene in redistricting cases, emphasizing that legislative districts must be roughly equal in population, thus ensuring fair representation. This landmark decision significantly impacted political practices in the United States, promoting equitable voting rights and influencing redistricting efforts across the country.