Three historic Supreme Court Cases – all decided in the mid 60’s - are responsible for constitutionally mandated, periodic redistricting of political boundary lines; Baker v. Carr, Wesberry v. Sanders, and Reynolds v. Simms. In Baker, the court affirmed that cases regarding political “apportionment” were capable of being adjudicated in courts as they were not nonjusticiable political questions ill-suited for the judiciary.

In Wesberry, the court held that requirement in Article 1 Sec. 2 of the U.S. constitution that Representatives be chosen "by the People of the several States" means that, as nearly as is practicable, one person's vote in a congressional election is to be worth as much as another's. (376 U.S. 7-8). That case involved Georgia congressional districts that had not been redrawn in over 30 years. The largest congressional district in this case had a population of 823,680, and the smallest 272,154.

In Reynolds, the court held that the “Equal Protection Clause [of the 14^th Amendment] requires substantially equal legislative representation for all citizens in a State regardless of where they reside.” (377 U.S. 533, 561 (1964) That case involved Alabama state legislative districts that had not been rebalanced since 1901.

One Man, One Vote Principle

The court in these rulings recognized that votes that were not equally weighted, can and do distribute power unevenly.  The constitutional requirement for an equally weighted vote became known as the “one-man, one vote” principle.

Since 1964, redistricting became a constitutional requirement that occurs at least once every decade. States had traditionally used the decennial census as a starting point for redistricting even before these three foundational court decisions, but it was in response to these rulings that the regular ten-year redistricting cycle began for states after the 1970 census.

Equal Population Rules

Congressional and legislative maps have two different standards when it comes to complying with the court’s Equal Population requirement. Congressional districts have historically had a stricter standard (with some exceptions) that requires these districts to be “as nearly equal in population as practicable.” State legislative districts must be “substantially equal.”

These two standards may not sound very different, but the court has been explicit in subsequent opinions that congressional district population equality is a stricter standard because it comes from a literal directive in the constitution itself. The state legislative equality standard derives from the general terms of the Equal Protection Clause in the 14^th amendment. Because of this, in the court’s mind, there is more latitude when redrawing state legislative lines.