The U.S. Supreme Court case Kilgarlin et al. v. Hill, Secretary of State of Texas, et al., 1966 revolved around the issue of legislative redistricting in Texas and whether it violated the Equal Protection Clause under the Fourteenth Amendment due to population disparities among districts. The plaintiffs argued that their votes were diluted because they resided in heavily populated districts compared to other less-populated ones with equal representation at state level - a principle known as "one person, one vote". The District Court initially dismissed this claim stating that there was no constitutional requirement for both houses of a bicameral legislature to be apportioned on a population basis. However, upon appeal by plaintiffs, the Supreme Court reversed this decision citing its ruling from Reynolds v Sims (1964) which established 'one-person-one-vote' rule for state legislative districting matters too and not just federal elections or lower house seats alone as earlier understood by some states including Texas. It held that both chambers must have roughly equal populations within each respective district so every citizen's voting power is approximately equivalent across entire state irrespective of where they live.

In the dissenting opinion for Kilgarlin et al. v. Hill, Justice John Marshall Harlan II argued that the Court was overstepping its boundaries by intervening in a matter of state legislative apportionment, which he believed should be left to political processes and not judicial review. He contended that there were no clear constitutional guidelines or standards for determining when a state's apportionment plan becomes so unfair as to violate equal protection principles. Furthermore, he expressed concern about the potential consequences of such intervention on federalism and separation of powers principles, cautioning against an overly broad interpretation of judicial power under the Fourteenth Amendment's Equal Protection Clause.