The decennial census has played a crucial role in the apportionment of the Congress for more than two centuries. But it is only in the last 35 years that the Census Bureau has played a major role in the redistricting process.

U.S. Supreme Court decisions handed down during the 1960s clarified the Constitution’s intention to provide equality of representation for all Americans. In 1964, the Wesberry v. Sanders decision held that, “as nearly as is practicable one person’s vote in a congressional election is to be worth as much as another’s.” That same year, in Reynolds v. Sims, the Court ruled that state legislative districts must be “as nearly of equal population as is practicable.”

Both U.S. congressional districts and state legislative districts must be drawn so that their residents have a fair and equal share in the way they are governed. These Supreme Court decisions increased the states’ need for geographically detailed census information in the redistricting process.

The urgency of the states’ need for these data led the Congress to pass Public Law (P.L.) 94-171 in December 1975.