Reynolds v. Sims (1964)

By the early 1960s many state legislatures had not redrawn their districts in decades, leaving growing cities badly underrepresented. Alabama's legislature was still based on the 1900 census, and some rural senate districts had a small fraction of the population of urban ones. After Baker v. Carr in 1962 held that such claims could be heard in federal court, Alabama voters challenged the apportionment of both legislative chambers.

On June 15, 1964, the Supreme Court ruled 8 to 1, in an opinion by Chief Justice Earl Warren, that the Equal Protection Clause requires seats in both houses of a state legislature to be apportioned substantially by population. Legislators represent people, not trees or acres, Warren wrote, rejecting the argument that a state senate could mirror the U.S. Senate by representing counties. Justice John Marshall Harlan dissented, arguing that the Constitution left apportionment to the states. Earlier that year, in Wesberry v. Sanders, the Court applied the same principle to congressional districts.

The ruling forced nearly every state to redraw its legislature, shifting power from rural areas to cities and suburbs. Warren later called the apportionment cases the most important of his tenure. Courts allow small population deviations in state legislative districts but require near-exact equality for congressional districts. In Evenwel v. Abbott in 2016, the Court held that states may count total population rather than only eligible voters.