Gerrymandering is the deliberate drawing of electoral district boundaries to give one political party or racial group an unfair advantage. Every state has to redraw its congressional and legislative districts after each decennial census so that each district holds roughly the same number of people, and that redrawing crosses into gerrymandering when the people holding the pen design the lines to predetermine who wins. Because the resulting maps stay in place for a full decade, a single round of line-drawing can shape which party controls a legislature, and which communities get represented, ten elections in a row.
How the Line-Drawing Happens
The U.S. Census Bureau counts the national population every ten years, as required by Article I, Section 2 of the Constitution.1U.S. Census Bureau. About the Decennial Census of Population and Housing Those numbers show where people have moved, and states then redraw their districts to keep populations roughly equal. The Supreme Court set that equal-population rule in Wesberry v. Sanders (1964), holding that “as nearly as is practicable, one man’s vote in a congressional election is to be worth as much as another’s.”2Justia U.S. Supreme Court Center. Wesberry v. Sanders, 376 U.S. 1 (1964) Congressional districts must hit near-exact equality; state legislative districts get slightly more slack under the 14th Amendment’s Equal Protection Clause.3Congress.gov. ArtI.S2.C1.1 Congressional Districting
In most states, the state legislature draws the new maps. About a dozen states hand part or all of that job to a commission designed to limit partisan influence. After the 2020 census, six states gained congressional seats and seven lost them, which raised the stakes of the maps drawn in those states.4Congress.gov. ArtI.S2.C3.1 Enumeration Clause and Apportioning Seats in the House of Representatives
Legal redistricting is supposed to follow a handful of neutral principles. Districts should be compact, meaning shaped reasonably rather than sprawling in tentacles across the state. They should be contiguous, meaning every piece is physically connected. They should respect communities of interest, keeping neighborhoods, cities, and regions with shared concerns together. And they should follow existing county and city lines where practical. When a district violates most of those principles at once, that is the visual signature that something else is driving the map.
Cracking and Packing
Two techniques do almost all the work of a gerrymander: cracking and packing.
Cracking splits a bloc of like-minded voters across multiple districts so they never form a majority anywhere. A city of 100,000 voters who lean heavily one way can be sliced into four pieces, each attached to surrounding rural areas, so that those urban voters are outnumbered in every district they end up in.
Packing does the reverse. It stuffs as many opposing voters as possible into a single district, so that group wins one seat by an enormous margin and loses everything around it. A district where one party wins with 85 percent of the vote is almost certainly packed. Every vote beyond the bare majority needed to win is a vote that could have helped elsewhere and didn’t.
That is the concept of “wasted votes,” and it is the engine of both techniques. Any vote for a losing candidate is wasted, and so is any vote for a winning candidate beyond what they actually needed. Cracking wastes votes by scattering them into losses across many districts. Packing wastes them by piling up unnecessary wins in one. Either way, the party drawing the map converts a modest statewide vote share into a lopsided seat advantage.
Modern software has made this precise to the block level. Mapmakers feed in voter registration records, past election results, and demographic data, generate thousands of candidate maps, and pick the one that maximizes advantage. The output is districts with strange shapes that snake through neighborhoods to pick up favorable precincts and drop unfavorable ones. General elections in those districts are rarely competitive, so the real contest shifts to primaries, where candidates face pressure to appeal to their party’s base rather than the broader electorate.
Partisan Gerrymandering vs. Racial Gerrymandering
Gerrymandering comes in two flavors, and the law treats them very differently.
Partisan gerrymandering targets voters based on how they vote. The party controlling redistricting draws safe districts for its own candidates and forces the other party’s voters into a small number of districts they win by overwhelming margins.
Racial gerrymandering targets voters based on race or ethnicity, though in practice race and party often overlap. The same cracking-and-packing playbook applies: split a minority community across several districts to keep it from electing a preferred candidate, or pack it into one district to contain its influence to a single seat.
A related concept is the majority-minority district, where a racial or ethnic group makes up more than half the voting-age population. The Voting Rights Act sometimes requires such districts to give minority communities a real opportunity to elect representatives of their choice. But the same tool can be turned against those voters: packing them into one overwhelmingly minority district neutralizes their influence on every other district on the map.
What Federal Law Prohibits
Federal law reaches racial gerrymandering aggressively and partisan gerrymandering barely at all.
Section 2 of the Voting Rights Act of 1965 (52 U.S.C. § 10301) prohibits any voting practice that results in minority voters having less opportunity to participate in the political process or elect candidates of their choice.5Office of the Law Revision Counsel. 52 USC 10301 – Denial or Abridgement of Right to Vote on Account of Race or Color It is a results test. A map can violate Section 2 without any proof of discriminatory intent.
The 14th Amendment’s Equal Protection Clause limits how far mapmakers can rely on race. If race is the predominant factor in the design of a district, courts apply strict scrutiny, meaning the state must show a compelling interest and a narrowly tailored map.6Congress.gov. Amdt14.S1.8.6.6 Racial Vote Dilution and Racial Gerrymandering The 15th Amendment separately prohibits denial of voting rights on account of race.7Congress.gov. Amdt15.S1.4 Racial Gerrymandering and Right to Vote Clause
When minority voters challenge a map under Section 2, courts apply a three-part test from the Supreme Court’s 1986 decision in Thornburg v. Gingles. Challengers must show:8Justia U.S. Supreme Court Center. Thornburg v. Gingles, 478 U.S. 30 (1986)
- The minority group is large enough and geographically concentrated enough to form a majority in a single district.
- The group tends to vote as a cohesive bloc.
- The white majority typically votes as a bloc in a way that defeats the minority group’s preferred candidates.
If all three are met, the court then looks at the totality of circumstances, including any history of discrimination in the state, to decide whether the map violates the law. In 2023, the Supreme Court reaffirmed both Section 2 and the Gingles framework in Allen v. Milligan, rejecting Alabama’s argument that redistricting challenges should be evaluated using race-neutral computer-generated maps.9Justia U.S. Supreme Court Center. Allen v. Milligan, 599 U.S. ___ (2023)
The Rucho Gap: Federal Courts Won’t Touch Partisan Maps
Purely partisan gerrymanders are a different story. In Rucho v. Common Cause (2019), the Supreme Court ruled 5–4 that partisan gerrymandering claims are “political questions beyond the reach of the federal courts.”10Justia U.S. Supreme Court Center. Rucho v. Common Cause, 588 U.S. ___ (2019) The majority acknowledged that extreme partisan gerrymandering may be “incompatible with democratic principles” but concluded there is no “judicially discernible and manageable” standard for saying when partisan line-drawing goes too far.11Congressional Research Service. Partisan Gerrymandering Claims Not Subject to Federal Court Review: Considerations Going Forward
The practical effect is enormous. If a party in power draws a map that hands it 70 percent of the seats on 55 percent of the statewide vote, no federal lawsuit can undo it on partisan grounds alone. A challenger has to find a racial discrimination angle that triggers Section 2 or Equal Protection review, or take the case to state court.
State Courts as the Remaining Check
After Rucho shut the federal courthouse door on partisan claims, state courts became the main venue. About 30 state constitutions have clauses requiring elections to be “free,” and roughly 18 of those go further and require elections be “free and equal” or “free and open.” Those provisions give state courts an independent legal basis to review whether a legislature’s map is too skewed.
Several state supreme courts have used them. In 2022, North Carolina’s supreme court relied on the state’s “free elections” clause to strike down the legislature’s proposed congressional map as excessively partisan. New York’s highest court threw out maps that same year for violating state constitutional redistricting procedures, including requirements added by a 2014 reform amendment. Pennsylvania’s supreme court had already used its “free and equal elections” clause in 2018 to throw out and replace the state’s congressional map.
These challenges are not guaranteed to succeed. Not every state constitution has a clause strong enough to support the claim, and state courts vary widely in how willing they are to step in. But state court litigation is now the most viable path for voters trying to challenge a partisan map.
How Anyone Knows a Map Is Rigged
Courts and political scientists use quantitative tools to detect manipulation. Two of the most discussed are the efficiency gap and the mean-median difference.
The efficiency gap counts wasted votes on each side. It subtracts one party’s wasted votes from the other’s and divides by total votes cast. A large gap suggests one party is systematically converting votes into seats more efficiently than the other, which is the fingerprint of cracking and packing. The metric featured heavily in the Wisconsin case Whitford v. Gill, though the Supreme Court never endorsed it as a legal standard before Rucho made the question moot in federal court.
The mean-median difference compares a party’s average vote share across all districts to its share in the median district. In a balanced map, those numbers should be close. When they diverge, one party needs to win a significantly larger share of the statewide vote just to capture half the seats.
Neither metric alone proves anything, and federal courts have declined to adopt any single formula as a legal test. They are strongest when combined with visual analysis of district shapes and with comparisons to thousands of computer-generated alternative maps.
Reform and Public Participation
Redistricting reform has moved mainly through ballot initiatives. Voters in Arizona, California, Michigan, Colorado, and Utah have approved measures creating independent or bipartisan commissions that take direct map-drawing out of legislators’ hands. The commissions vary in structure. Some are fully independent, others advisory, but the shared goal is to reduce the conflict of interest in letting elected officials choose their own voters.
Public participation during the process itself also matters. Most states hold hearings before adopting new maps, and written or spoken testimony at those hearings becomes part of the record courts later review if the map is challenged. Pushing for explicit criteria — compactness, contiguity, respect for existing communities — gives courts clearer benchmarks when they have to decide whether a particular map has gone too far.