The Supreme Court has placed two firm constitutional limits on how states draw legislative districts and declined to impose a third. Districts must contain roughly equal populations, and race cannot be the dominant reason for a district’s boundaries. Beyond those two rules, the Supreme Court’s limits on gerrymandering stop: partisan map-drawing, however extreme, is outside the reach of federal courts. That gap matters, because partisan manipulation is the most common form of gerrymandering and now has no federal judicial remedy.
Districts Must Have Roughly Equal Populations
The oldest limit is mathematical. In Reynolds v. Sims (1964), the Court held that state legislative districts must contain roughly equal populations, with Chief Justice Earl Warren writing that “legislators represent people, not trees or acres.”1Justia U.S. Supreme Court Center. Reynolds v. Sims, 377 U.S. 533 (1964) The same year, Wesberry v. Sanders applied a parallel rule to congressional districts, requiring that “as nearly as is practicable one man’s vote in a congressional election is to be worth as much as another’s.”2Library of Congress. Wesberry v. Sanders, 376 U.S. 1 (1964) Together they became known as “one person, one vote.”
The rule is narrow. Equal population prevents one type of vote dilution and nothing more. A state can draw wildly contorted districts that heavily favor one party, and as long as the population counts line up, the constitutional floor is satisfied. States may also use total population, not just eligible voters, as the baseline. The Court confirmed that choice in Evenwel v. Abbott (2016), reasoning that representatives serve everyone in their districts.3Justia U.S. Supreme Court Center. Evenwel v. Abbott, 578 U.S. ___ (2016)
Race Cannot Be the Predominant Factor
The Court’s most active policing of district lines involves race. When race is the dominant reason for a district’s shape, the map triggers strict scrutiny, and the state must prove a compelling justification and a narrowly tailored design.
Shaw v. Reno (1993) set the framework. North Carolina had drawn a congressional district stretching roughly 160 miles along Interstate 85, in places no wider than the highway corridor. The Court held that when a district’s shape is so bizarre it can only be understood as an effort to sort voters by race, plaintiffs have stated a valid equal protection claim.4Justia U.S. Supreme Court Center. Shaw v. Reno, 509 U.S. 630 (1993) If traditional factors like compactness, community ties, or existing political boundaries explain the shape, the district can survive; if race overrides those factors, the state must justify what it did.5Constitution Annotated. Racial Vote Dilution and Racial Gerrymandering
Proving a Shaw claim means showing that race was the predominant factor driving the mapmaker’s decisions, overriding those traditional criteria. Courts evaluate this district by district, not by looking at the statewide map as a whole.6Legal Information Institute. Racial Vote Dilution and Racial Gerrymandering
The Voting Rights Act Pulls in the Opposite Direction
The Equal Protection Clause forbids making race the predominant factor. Section 2 of the Voting Rights Act, however, can require states to account for race when drawing maps. Section 2 bars any voting practice that gives minority voters less opportunity than others to elect candidates of their choice, which in practice sometimes forces a state to draw a majority-minority district to avoid diluting minority voting strength.7Office of the Law Revision Counsel. 52 USC 10301 – Denial or Abridgement of Right to Vote on Account of Race or Color
The threshold test comes from Thornburg v. Gingles (1986). A challenger must show three things: the minority group is large enough and geographically compact enough to form a majority in a single district; the group votes cohesively; and the white majority votes as a bloc in a way that usually defeats the minority group’s preferred candidates.8Justia U.S. Supreme Court Center. Thornburg v. Gingles, 478 U.S. 30 (1986) When all three conditions are met, failing to draw a majority-minority district can violate federal law.
The Court reaffirmed that framework in Allen v. Milligan (2023), finding that Alabama’s congressional map likely violated Section 2 by splitting a compact Black population across multiple districts instead of drawing a second majority-Black district. The majority stressed that a Gingles challenger must offer an alternative map respecting traditional criteria like compactness and county boundaries, not just raw population numbers.9Justia U.S. Supreme Court Center. Allen v. Milligan, 599 U.S. ___ (2023)
Map drawers have to thread the needle. Comply too aggressively with Section 2 and a state risks a Shaw claim; ignore it and the state risks a vote dilution suit. Race can be one factor. It cannot become the predominant one.
When Race and Partisanship Overlap
In most of the country, racial demographics and partisan voting patterns are closely correlated. A district drawn to pack Democratic voters may also pack Black voters, and separating the two motives is often difficult.
The Court addressed that problem in Alexander v. South Carolina State Conference of the NAACP (2024). The majority held that when race and politics are intertwined, a challenger must disentangle them and prove that race, not partisanship, was the legislature’s dominant motive. Courts start with a presumption that the legislature acted in good faith, and if either race or politics could explain a district’s boundaries, the challenger has not carried the burden.10Supreme Court of the United States. Alexander v. South Carolina State Conference of the NAACP
The decision also made alternative maps close to mandatory. A plaintiff who fails to submit a map showing that a legislature pursuing its stated partisan goals could have achieved greater racial balance invites an adverse inference that may decide the case on its own.10Supreme Court of the United States. Alexander v. South Carolina State Conference of the NAACP For jurisdictions where race and party affiliation overlap heavily, Alexander raised the practical bar for racial gerrymandering claims significantly.
Partisan Gerrymandering Is Outside Federal Courts
The Court’s most consequential redistricting decision may be the one where it refused to act. In Rucho v. Common Cause (2019), a 5-4 majority ruled that partisan gerrymandering claims are political questions federal courts cannot resolve.11Justia U.S. Supreme Court Center. Rucho v. Common Cause, 588 U.S. ___ (2019)
The cases before the Court involved maps openly designed for partisan advantage. North Carolina’s congressional map was drawn by Republican legislators who publicly stated their goal of electing ten Republicans and three Democrats. Maryland’s map was redrawn by Democrats to flip a historically Republican seat. Chief Justice Roberts, writing for the majority, called excessive partisan gerrymandering “incompatible with democratic principles” but concluded that the Constitution offers no workable standard for judges to decide when a map crosses from acceptable political consideration into unconstitutional manipulation.12Supreme Court of the United States. Rucho v. Common Cause (2019)
Roberts distinguished the two areas where federal courts still intervene. Population equality has a clear mathematical standard. Racial gerrymandering has an established doctrinal framework. Partisanship, he wrote, would require judges to decide how much is too much, and any answer would inevitably reflect judicial preferences about election outcomes.
The practical impact is large. Map drawers in states controlled by one party can use detailed data and software to lock in advantages for a decade. The two standard techniques are splitting disfavored voters across many districts so they lack a majority anywhere, and concentrating them into a few districts so their votes are wasted by overwhelming margins. Both are lawful at the federal level as long as the map satisfies population equality and does not use race as the predominant factor.
State Courts and Commissions Are the Remaining Avenue
The Rucho majority was explicit that its ruling did not put partisan gerrymandering beyond all legal challenge. The opinion pointed to state constitutional provisions, independent redistricting commissions, and legislation mandating nonpartisan criteria.12Supreme Court of the United States. Rucho v. Common Cause (2019)
Many state constitutions guarantee “free and equal” or “free and open” elections with no federal counterpart. State supreme courts in Alaska, Florida, Maryland, New York, Ohio, and Pennsylvania have invalidated gerrymandered maps under those provisions. Pennsylvania’s Supreme Court struck down the state’s congressional map under its Free and Equal Elections Clause, holding that subordinating neutral redistricting criteria to partisan advantage violated the state constitution.
The Supreme Court preserved that avenue in Moore v. Harper (2023). Some legislators had argued, under the “independent state legislature” theory, that the federal Elections Clause gives state legislatures exclusive power over federal election rules, free from state court oversight. The Court rejected that argument, holding that state legislatures remain subject to their own constitutions and ordinary state judicial review when they draw congressional maps.13Supreme Court of the United States. Moore v. Harper (2023)
Roughly a dozen states have moved map-drawing authority to a commission. Structures vary from advisory panels to independent bodies with final authority over the lines. The Supreme Court upheld the strongest version of that approach in Arizona State Legislature v. Arizona Independent Redistricting Commission (2015), ruling 5-4 that the Elections Clause allows a state’s voters to assign redistricting to an independent body through a ballot initiative.
The Bottom Line
Federal courts enforce two boundaries on redistricting: districts must have roughly equal populations, and race cannot be the predominant factor in the lines. Section 2 of the Voting Rights Act adds a requirement that can pull the other way, sometimes compelling states to consider race to avoid diluting minority voting power. Alexander (2024) made racial claims harder to win when partisanship offers an alternative explanation. Rucho (2019) placed partisan gerrymandering entirely outside federal judicial reach. For voters in states with aggressive partisan maps and no state constitutional remedy, the Court has said plainly that the fix, if it comes, will be political rather than judicial.