Congressional district boundaries are drawn by each state after every ten-year census, usually by the state legislature and in a growing number of states by a redistricting commission. Whoever holds the pen has to divide the state into districts of nearly equal population and avoid diluting the voting power of racial minorities under federal law. Beyond those two hard federal rules, most of what shapes a map — the software, the hearings, the political horse-trading, the treatment of counties and cities — is set by state law and varies dramatically from one state to the next.
The Census Trigger
Redistricting starts with the count. Article I, Section 2 of the Constitution requires a population enumeration every ten years, and that count determines how the 435 seats in the House of Representatives are divided among the states.1Legal Information Institute. US Constitution Article I Once the President transmits the apportionment results to Congress, the Clerk of the House sends each state a certificate showing its new seat count.2Office of the Law Revision Counsel. 2 USC 2a – Reapportionment of Representatives
States that gained or lost seats obviously need new maps. But even a state whose seat count didn’t change has to redraw, because populations shift within the state over a decade and the districts have to stay equal in size. The Census Bureau is required by law to deliver the detailed population data states need for that work by April 1 of the year after the census; for the 2030 count, that deadline is April 1, 2031.3United States Census Bureau. Redistricting Data Program Management The package includes population counts down to the census block level, which is the raw material every map-drawer works from.
Who Actually Draws the Maps
The Constitution leaves redistricting to the states, and states have made very different choices about who does the drawing. In most states, the legislature draws congressional maps the same way it passes any other bill: a committee drafts a proposal, both chambers vote, and the governor can sign or veto it. That arrangement gives the party in power enormous influence over the shape of districts for the next decade.
About eleven states now use some form of commission for congressional redistricting. These range from citizen-led bodies that exclude elected officials to bipartisan panels split evenly between the two major parties, often with a neutral tiebreaker. Advisory commissions sit in the middle: an independent body drafts maps, but the legislature keeps the final vote. The structure matters because it changes the incentives. A legislature-controlled process almost guarantees the majority party will draw favorable lines; an independent commission at least breaks that link, even if it doesn’t take politics out of the room.
Equal Population
The most fundamental legal requirement is population equality. In Wesberry v. Sanders, the Supreme Court read Article I, Section 2 to mean that “as nearly as is practicable, one man’s vote in a congressional election is to be worth as much as another’s.”4Justia. Wesberry v. Sanders, 376 US 1 (1964) The Fourteenth Amendment’s Equal Protection Clause reinforces the principle for districts within a state.5Legal Information Institute. US Constitution Annotated – Voting Rights Generally
For congressional maps the tolerance is very tight. Courts have struck down maps where districts differed by less than one percent when the state could not justify the gap. State legislative districts get slightly more leeway, but congressional lines face the strictest population standard in American redistricting law. Any deviation has to be defended by a legitimate, consistently applied state policy, and even then the numbers have to stay as close to mathematical equality as possible.
Race and the Voting Rights Act
Section 2 of the Voting Rights Act prohibits any voting practice that “results in a denial or abridgement of the right of any citizen of the United States to vote on account of race or color.”6Office of the Law Revision Counsel. 52 USC 10301 – Denial or Abridgement of Right to Vote A violation is shown when, looking at the totality of circumstances, a minority group has less opportunity than other voters to participate in the political process and elect its preferred candidates. Plaintiffs do not have to prove discriminatory intent. Effect can be enough.
To bring a successful Section 2 challenge against a redistricting plan, a plaintiff first has to satisfy three preconditions the Supreme Court set out in Thornburg v. Gingles. The minority group must be large enough and geographically compact enough to form a majority in a reasonably drawn district. It must be politically cohesive, meaning its members tend to support the same candidates. And the white majority must vote as a bloc in a way that usually defeats the minority group’s preferred candidates.7Justia. Thornburg v. Gingles, 478 US 30 (1986) If the three preconditions are met, the court then looks at the broader circumstances to decide whether the political process is genuinely unequal for that group.
The Supreme Court reaffirmed the strength of Section 2 in Allen v. Milligan in 2023, holding that Alabama’s congressional map likely violated the statute because it failed to create a second district where Black voters could elect their preferred candidate. The Court rejected arguments that Section 2 requires proof of discriminatory intent or is unconstitutional as applied to redistricting, and it confirmed that the Gingles framework still governs.8Justia. Allen v. Milligan, 599 US (2023)
Before 2013, the Voting Rights Act had a second layer. Section 5 required states and counties with a history of racial discrimination to get federal approval before making any changes to voting rules, including new district maps. The Supreme Court effectively suspended that requirement in Shelby County v. Holder by striking down the formula used to identify covered jurisdictions. Section 5 itself was not held unconstitutional, but with no valid coverage formula, no jurisdiction is currently required to seek preclearance.9United States Department of Justice. About Section 5 of the Voting Rights Act States that once needed federal sign-off now enact new maps immediately, with Section 2 lawsuits after the fact as the main check on racial discrimination in redistricting.
The Line Between Using Race and Being Driven by It
Map-drawers have to consider race enough to comply with the Voting Rights Act, but the Equal Protection Clause forbids them from making race the dominant factor in how they draw lines. The Supreme Court set that boundary in Shaw v. Reno, holding that a plan so “bizarre on its face that it is unexplainable on grounds other than race” triggers strict judicial scrutiny; the map survives only if it is narrowly tailored to a compelling government interest.10Justia. Shaw v. Reno, 509 US 630 (1993)
Proving a racial gerrymandering claim has gotten harder. In Alexander v. South Carolina State Conference of the NAACP in 2024, the Court stressed that legislatures get a presumption of good faith and that showing race predominated requires clearing a high bar. The Court was particularly wary of cases where race and partisan preference correlate closely, warning that inferring racial motive from the effects of a political gerrymander would undermine the Rucho decision keeping partisan gerrymandering claims out of federal court.11Supreme Court of the United States. Alexander v. South Carolina State Conference of the NAACP (2024)
Partisan Gerrymandering
Drawing lines to favor a political party is the most visible redistricting fight, and federal law gives it the least attention. In Rucho v. Common Cause, the Supreme Court held in 2019 that partisan gerrymandering claims are political questions “beyond the reach of the federal courts.”12Justia. Rucho v. Common Cause, 588 US (2019) The majority acknowledged that extreme partisan gerrymandering “reasonably seems unjust” but said the remedy lies with Congress under the Elections Clause, not with federal judges.13Legal Information Institute. US Constitution Annotated – Partisan Gerrymandering
With the federal courthouse door closed, state courts have become the primary battleground. New York’s Court of Appeals invalidated maps drawn with “impermissible partisan purpose” and ordered a special master to draw replacements. North Carolina’s supreme court initially struck down gerrymandered maps under the state’s free elections clause, then reversed itself after a change in the court’s partisan composition and held that the same provisions do not prohibit partisan gerrymandering. Protection against partisan line-drawing now varies wildly by state and depends heavily on the composition of each state’s judiciary.
Traditional Districting Criteria
Beyond the federal mandates, most states apply a set of traditional principles meant to keep districts sensible for the people living in them. These are not federal requirements. They are state rules or customs, and states weight them differently.
- Contiguity: every part of a district must be physically connected, so you can travel from any point in the district to any other without leaving its boundaries. Nearly every state requires this.
- Compactness: districts should have a reasonably regular shape rather than sprawling, snaking, or jutting out in odd directions. Few states define the term precisely.
- Preserving political subdivisions: keeping counties, cities, and towns inside a single district rather than splitting them, which reduces confusion for voters and local election officials.
- Communities of interest: keeping together groups of people who share economic, social, or cultural ties so they can advocate through a single representative. What counts is inherently subjective, which makes the criterion both flexible and easy to manipulate.
These principles routinely conflict with each other and with the federal rules on equal population and racial fairness. When they do, the federal rules win. A district that perfectly preserves county boundaries but has 50,000 fewer people than its neighbor will be struck down no matter how tidy it looks.
How a Map Becomes Law
Once map-drawers have a draft, adoption typically involves public input and formal votes before the map takes legal effect. In legislature-controlled states the process mirrors ordinary legislation: committees develop the proposal, both chambers debate and amend, and a majority vote in each chamber sends it to the governor. The governor can sign or veto, and legislatures in most states can override a veto, though the threshold ranges from a simple majority to a two-thirds supermajority.
Commission states follow their own procedures, which often include stronger transparency requirements. Many commissions must hold public hearings before finalizing a map, and the number of required hearings ranges from none to more than twenty. Some commissions require a supermajority vote to certify the final plan, which forces members from both parties to agree before a map can move forward.
Public hearings tend to follow a predictable pattern. Residents and advocacy groups flag communities that proposed lines would split, point out oddly shaped districts, and raise racial or partisan fairness concerns. How much weight that testimony carries depends on who controls the process. In legislature-driven systems the majority party has made most of the key decisions before the first hearing begins; commission-based systems give the public a more genuine role, though commissions face political pressures of their own. After the responsible body approves the map, it is filed with the state’s chief election official and used for the rest of the decade.
When Courts Redraw the Map
If a state’s map violates federal law or the state constitution, courts can impose their own plan. When a court finds a violation, it typically gives the legislature or commission a deadline to draw a remedial map. If that deadline passes without an acceptable replacement, the court appoints a special master to draw one.
Recent cycles have produced a wave of court-drawn maps. After Allen v. Milligan, Alabama’s legislature failed to produce a remedial congressional map that satisfied the court’s order, and a special master redrew the state’s districts for the 2024 elections.8Justia. Allen v. Milligan, 599 US (2023) New York’s Court of Appeals ordered a special master to draw replacement maps after finding impermissible partisan purpose. Courts in North Carolina, Mississippi, Washington, and Wisconsin have all imposed maps in the 2020s after legislatures or commissions failed to produce lawful alternatives.
Court-drawn maps tend to be more politically neutral than legislature-drawn maps, partly because special masters prioritize traditional criteria like compactness and preserving political subdivisions over partisan advantage. But court involvement also means the elected branches lose control of a process the Constitution assigns to them, so courts generally treat map-drawing as a last resort.
Can Lines Change Between Censuses?
Most people assume district lines are locked for a full decade, and in practice they usually are. Federal law does not actually prohibit a state from redrawing its maps between censuses. The Supreme Court addressed the question in League of United Latin American Citizens v. Perry in 2006 and found no constitutional barrier to mid-decade congressional redistricting. Whether a specific state can do it turns on state law.
A handful of states explicitly prohibit mid-decade congressional redistricting. Others expressly allow it: Washington permits its legislature to revisit redistricting at any time, and Ohio requires mid-decade redistricting for congressional maps that did not pass with a specified level of bipartisan support. Most states simply don’t address the question, which leaves the legality uncertain until a court weighs in. Mid-cycle redistricting stays rare, but a ten-year-old map is not always guaranteed to survive the full ten years.