What happens after the census?

After the census, congressional districts are redrawn if a state has gained or lost House seats, and even if the number of seats stays the same, districts within each state must be redrawn to reflect population shifts so each district contains roughly equal numbers of people. This process is called redistricting. The constitutional basis for redistricting comes from the requirement in Article I, Section 2 that representatives be apportioned among the states based on population. The Supreme Court added the principle of one person, one vote in Wesberry v. Sanders (1964), holding that congressional districts within a state must contain roughly equal populations to ensure that each person's vote carries equal weight. Reynolds v. Sims (1964) extended this principle to state legislative districts.

After each census, the Census Bureau provides population data to the states. The federal government determines how many House seats each state will have for the coming decade through reapportionment, recalculating the distribution of the 435 seats among the states. States that gained or lost seats must redraw their congressional districts to reflect the new number. Even states whose total seats did not change must redistrict because populations shift within states between censuses. Some areas grow faster than others, requiring district lines to move to maintain roughly equal populations.

Who controls redistricting varies by state. In most states, redistricting is controlled by the state legislature, which passes a redistricting map as ordinary legislation, with the governor able to sign or veto. In states with one-party control of state government, the controlling party often draws maps that favor its candidates, a practice called partisan gerrymandering. Some states use independent redistricting commissions designed to reduce partisan influence. California, Arizona, Michigan, Colorado, and others use independent commissions composed of citizens from both parties and independents. Iowa uses a nonpartisan staff to draw initial maps that the legislature must accept or reject without amendments.

Federal law also imposes requirements on redistricting under the Voting Rights Act of 1965. Section 2 of the act prohibits voting practices, including district lines, that have a discriminatory effect on minority voters. The Supreme Court has generally upheld these requirements, though specific applications have been heavily contested. The Court has also held that partisan gerrymandering claims are not justiciable in federal court (Rucho v. Common Cause, 2019), meaning federal courts cannot strike down maps for being too partisan, though state courts can do so under state constitutions. Recent redistricting cycles have seen significant litigation in many states.