Gerrymandering Explained: How District Lines Shape Elections

Gerrymandering is the practice of drawing the boundaries of electoral districts to favor a particular group, party, or outcome. Because where the lines are drawn helps determine who wins, redistricting is one of the most consequential — and most debated — parts of the American election system.

What Is Gerrymandering?

The United States elects most of its legislators from geographic districts. Each U.S. House district, and each state legislative district, is supposed to represent roughly equal numbers of people. Someone has to decide exactly where those boundaries go, and small choices about which neighborhoods are grouped together can change which candidates are likely to win.

Gerrymandering refers to drawing those boundaries to produce a desired political result rather than to reflect natural communities. The term dates to 1812, when a Massachusetts district signed into law under Governor Elbridge Gerry was said to resemble a salamander — hence "Gerry-mander." The practice is far older than the word, and over the centuries it has been used by figures across the political spectrum.

It is worth being precise: drawing districts is a normal, required part of government. The controversy is about how and why the lines are drawn, and whether a given map crosses a line from ordinary line-drawing into manipulation.

Two Main Types: Partisan and Racial Gerrymandering

Courts and analysts generally distinguish between two kinds of gerrymandering, which are treated very differently under the law.

Partisan gerrymandering

This is drawing maps to advantage one political party over another — for example, to help one party win more seats than its share of the statewide vote might otherwise suggest. Critics argue this lets officeholders effectively choose their voters instead of the other way around. Defenders sometimes argue that some degree of political consideration is unavoidable and that mapmakers chosen through elections have a legitimate role.

Racial gerrymandering

This involves drawing district lines based on race in ways that dilute or distort the voting strength of racial or language-minority groups. Unlike partisan questions, race-based vote dilution is directly addressed by federal law, principally the Voting Rights Act of 1965 (VRA). Section 2 of the VRA prohibits voting practices, including district maps, that discriminate on the basis of race. At the same time, the Supreme Court has held that race cannot be the predominant factor in drawing a district without strong justification. The result is a genuinely difficult legal balancing act, and courts continue to work through where the boundaries lie.

How Map-Drawers Do It: Packing and Cracking

Two techniques are described again and again in court cases and academic studies.

• Packing: Concentrating as many of a particular group's voters as possible into a small number of districts. Those districts are won by overwhelming margins, but the group's overall influence is limited because its votes are "wasted" piling up lopsided wins in just a few places.
• Cracking: Splitting a group's voters across many districts so that they fall just short of a majority in each one. This spreads the group thin so it cannot reliably elect a candidate of choice anywhere.

Used together, packing and cracking can let mapmakers shape likely outcomes without changing a single vote. Analysts sometimes measure the effect using metrics such as "wasted votes" or the "efficiency gap," though experts disagree about how reliable any single measure is.

How Redistricting Works After the Census

Gerrymandering happens during redistricting, the once-a-decade process of redrawing district lines. Here is the basic cycle.

• The census. The U.S. Constitution requires a count of the population every ten years. The most recent was in 2020; the next is scheduled for 2030.
• Apportionment. Census results determine how many of the 435 U.S. House seats each state receives. States that grow faster may gain seats; others may lose them.
• Redrawing the lines. Each state then redraws its congressional and legislative districts so they have roughly equal population, reflecting the Supreme Court's "one person, one vote" principle.

Who draws the maps varies a great deal by state. In many states the state legislature draws the lines, sometimes subject to a governor's veto. Others use independent or bipartisan commissions, advisory commissions, or backup commissions that step in if the legislature deadlocks. Courts may also become involved if maps are challenged. Because the rules differ so much, the only reliable way to know your state's process is to check with your state election office or Secretary of State.

Most states also apply traditional criteria such as keeping districts contiguous (connected), reasonably compact, and respectful of existing political boundaries and "communities of interest." How strictly these criteria apply is itself set by each state's constitution and statutes.

Independent Redistricting Commissions

One widely discussed reform is moving map-drawing out of the hands of elected legislators and into a commission. These bodies take different forms:

• Independent commissions made up of citizens who are not current officeholders.
• Bipartisan commissions with members appointed by both major parties.
• Advisory or backup commissions that propose maps or act only when the regular process stalls.

Supporters argue commissions reduce self-interested line-drawing and increase public trust. Skeptics question whether any body can be truly neutral, point to disputes over how members are chosen, and note that commission maps can still be challenged in court. Several states have adopted commissions through ballot initiatives; others have rejected or limited them. The Supreme Court has addressed the constitutionality of citizen commissions, upholding the use of an independent commission for congressional districts in Arizona State Legislature v. Arizona Independent Redistricting Commission (2015). Whether a commission is a good idea is a policy judgment reasonable people disagree about.

Key Court Rulings

The legal landscape is shaped by several important Supreme Court decisions. A few are especially central.

• Baker v. Carr (1962) and Reynolds v. Sims (1964) established that federal courts could hear redistricting cases and that districts must be roughly equal in population — the "one person, one vote" rule.
• Shaw v. Reno (1993) and Miller v. Johnson (1995) held that race cannot be the predominant factor in drawing a district without satisfying strict constitutional scrutiny.
• Rucho v. Common Cause (2019) is the pivotal recent ruling on partisan gerrymandering. The Court held that claims of purely partisan gerrymandering present "political questions" that are beyond the reach of the federal courts — meaning federal judges will not decide whether a map is too partisan. Importantly, the Court did not say partisan gerrymandering is good policy; it left the issue to be addressed by Congress, by the states, and through state courts and state constitutions.

The practical effect of Rucho is that much of the action on partisan maps has shifted to the states. Some state supreme courts have struck down maps under their own state constitutions, and some states have changed their processes through legislation or ballot measures. Federal courts still hear racial gerrymandering and Voting Rights Act claims, which remain an active and evolving area of law.

Why It Matters to Voters

District lines influence how competitive elections are, how responsive representatives feel to constituents, and how closely the makeup of a legislature tracks the overall vote. People disagree, sincerely and across party lines, about how much line-drawing is acceptable and who should do it. Those are exactly the kinds of questions voters can weigh in on — through ballot initiatives, state legislative elections, and public comment periods that many states hold during redistricting.

If you want to engage, you can look up your own district maps, find out who draws them in your state, and watch for public hearings. Your state election office or Secretary of State is the authoritative source for how redistricting works where you live and how to participate.

Bottom Line

Gerrymandering is the drawing of district lines to shape election outcomes, using techniques like packing and cracking. Racial gerrymandering is restricted by the Constitution and the Voting Rights Act and is reviewed by federal courts; partisan gerrymandering, after Rucho v. Common Cause, is left mainly to Congress and the states. Because rules and remedies vary widely from state to state, the best step you can take is to learn your own state's process from official sources and decide for yourself.