Redistricting in the United States is the process of drawing electoral district boundaries. A congressional act passed in 1967 requires that representatives be elected from single-member districts, except when a state has a single representative, in which case one state-wide at-large election be held.
Seven states have only a single representative in the United States House of Representatives for the entire state, because of their low populations. These are Alaska, Delaware, Montana, North Dakota, South Dakota, Vermont, and Wyoming. These states do not need redistricting for the House and elect members on a state-wide at-large basis.
In 25 states, the state legislature has primary responsibility for creating a redistricting plan, in many cases subject to approval by the state governor. To reduce the role that legislative politics might play, thirteen states (Alaska[a], Arizona, California, Colorado, Hawaii, Idaho, Michigan, Missouri, Montana[b], New Jersey, Ohio, Pennsylvania, and Washington) determine congressional redistricting by an independent or bipartisan redistricting commission. Five states: Maine, New York, Rhode Island, Vermont[c], and Virginia give independent bodies authority to propose redistricting plans, but preserve the role of legislatures to approve them. Arkansas has a commission composed of its governor, attorney general, and secretary of state.
By law, the 43 states with more than one Representative must redistrict after each decennial census to account for population shifts within the state as well as (when necessary) to add or remove congressional districts. States are not prevented from redistricting at any time between censuses up to and including redistricting prior to each congressional election, provided such redistricting conforms to various federal laws. However, "mid-decade" redistricting proposals (such as what occurred in 2003 in Texas) have typically been highly controversial. Another case of between-censuses redistricting occurred between the 2016 and 2018 elections, in Pennsylvania.
State constitutions and laws also mandate which body has responsibility over drawing the state legislature boundaries. In addition, those municipal governments that are elected on a district basis (as opposed to at-large) also redistrict.
The Reapportionment Act of 1929 withdrew the size and population requirements for Congressional districts, last stated in the Apportionment Act of 1911. The previous apportionment acts required districts be contiguous, compact, and equally populated.
Each state can set its own standards for Congressional and legislative districts. In addition to equalizing the population of districts and complying with federal requirements, criteria may include attempting to create compact, contiguous districts, trying to keep political units and communities within a single district, and avoiding the drawing of boundaries for purposes of partisan advantage or incumbent protection.
Redistricting may follow other criteria depending on State and local laws:
Gerrymandering, the practice of drawing district boundaries to achieve political advantage for legislators, involves the manipulation of district boundaries to leave out, or include, specific populations in a particular district to ensure a legislator's reelection or to advantage their party.
In states where the legislature (or another body where a partisan majority is possible) is in charge of redistricting, the possibility of gerrymandering (the deliberate manipulation of political boundaries for electoral advantage, usually of incumbents or a specific political party) often makes the process very politically contentious, especially when the majorities of the two houses of the legislature, or the legislature and the governor, are from different parties.
Partisan domination of state legislatures and improved technology to design contiguous districts that pack opponents into as few districts as possible have led to district maps which are skewed towards one party. Consequently, many states including Florida, Georgia, Maryland, Michigan, North Carolina, Ohio, Pennsylvania, Texas and Wisconsin have succeeded in reducing or effectively eliminating competition for most House seats in those states. Some states, including New Jersey and New York, protect incumbents of both parties, reducing the number of competitive districts.
The state and federal court systems are often involved in resolving disputes over Congressional and legislative redistricting when gridlock prevents redistricting in a timely manner. In addition, those disadvantaged by a proposed redistricting plan may challenge it in state and federal courts. Justice Department approval (which is known as pre-clearance) was formerly required under Section 5 of the Voting Rights Act of 1965 in certain states that have had a history of racial barriers to voting. The Supreme Court's ruling on the Pennsylvania redistricting effectively allows elected officials to select their constituents by eliminating most of the grounds for constituents to challenge district lines.