Redistricting

Redistricting is the process of drawing the boundaries of legislative districts. After the decennial census, states receive their apportionment of U.S. House seats and then draw new congressional and state legislative maps so each district has roughly equal population, a requirement of the "one person, one vote" principle from cases such as Reynolds v. Sims (1964) and Wesberry v. Sanders (1964).

Most states give the legislature the main role, with the governor possibly able to veto the map. Others use independent or bipartisan commissions, as in Arizona, California and Michigan. Courts review maps for equal population, compliance with the Voting Rights Act and, in the case of racial gerrymandering, the Constitution. State courts may enforce state rules requiring compactness or fairness.

Maps are normally changed once a decade, but a number of states have redrawn them mid-decade after court decisions or for partisan advantage, which has led to disputes. The process matters because maps influence which party controls the House and state legislatures. Conservatives and liberals disagree about the correct balance between considering race, compactness and political outcomes. See also gerrymandering.