Gerrymandering affects House elections by drawing district lines so one party wins more seats than its share of the vote would suggest. Mapmakers pack the other party’s voters into a few lopsided districts and crack the rest across many districts where they fall short. The result is fewer competitive seats, more decisive primaries, and a House majority that can depend as much on maps as on votes. That’s especially true in 2026, after an unusual wave of mid-decade redistricting.
How does gerrymandering work? #
Every district must have roughly the same population, so gerrymandering isn’t about how many people are in a district. It’s about which people. Mapmakers use two techniques.
Packing #
Pack as many of the other party’s voters as possible into a handful of districts. The other party wins those seats with 75% or 80% of the vote, and every vote beyond what it needed to win is wasted.
Cracking #
Split the other party’s remaining voters across several districts so they’re a minority in each. A city that could anchor one competitive seat gets divided into four slices, each attached to a larger rural or suburban area that outvotes it.
A simple example #
Imagine a state with five districts and 500 voters: 250 for Party A and 250 for Party B, a perfect tie.
| Map | District results | Seats A–B |
|---|---|---|
| Drawn by Party A | B packed into one district (90–10); A wins the other four 60–40 | 4–1 |
| Drawn by Party B | A packed into one district (90–10); B wins the other four 60–40 | 1–4 |
| Drawn for competitiveness | Several districts near 50–50 | Depends on the year |
Same voters, same votes, and the seat count swings from 4–1 to 1–4 depending on who holds the pen. Real maps are messier, but that’s the logic.
What does gerrymandering do to House elections? #
Fewer competitive districts #
When most seats are drawn to be safe, the general election is decided before anyone votes. In a typical recent cycle only a few dozen of the 435 seats are genuinely competitive, and the national majority is decided in those. That’s why race ratings outlets spend most of their effort on 30 to 50 districts.
The primary becomes the real election #
In a safe seat, the winner of the dominant party’s primary is almost certain to win in November. Primary electorates are smaller and more ideological, so members worry more about a primary challenger than about the other party. Many political scientists link this to polarization, though they debate how much, since the Senate (which can’t be gerrymandered) has polarized too.
Seats and votes drift apart #
A party can win a majority of a state’s House votes and a minority of its seats. Nationally, the effects of different states’ maps partly cancel out, but not always, and in a closely divided House a few seats of structural advantage can decide control.
Isn’t some of it just geography? #
Yes, and it’s worth separating the two. Democratic voters are heavily concentrated in cities, while Republican voters are spread more evenly across suburbs, towns and rural areas. Even a neutral map drawn for compact districts tends to pack urban Democrats into lopsided seats. Researchers use computer simulations that generate thousands of neutral maps to estimate how much of a map’s partisan tilt comes from geography and how much from deliberate line-drawing.
What happened with redistricting in 2025 and 2026? #
Normally states redraw once a decade, after the census. Starting in mid-2025, several states redrew their congressional maps again ahead of the 2026 midterms, with each side responding to the other.
| State | Action | Intended effect |
|---|---|---|
| Texas | Legislature passed a new map in August 2025; the Supreme Court allowed it to be used | Up to 5 more Republican-leaning seats |
| California | Voters approved Proposition 50 in November 2025, temporarily replacing the independent commission’s map | Up to 5 more Democratic-leaning seats |
| North Carolina | New map in October 2025 | 1 more Republican-leaning seat |
| Ohio | Commission adopted a new map in October 2025 | About 2 more Republican-leaning seats |
| Utah | Court ordered a new map in November 2025 under the state’s voter-approved anti-gerrymandering rules | 1 more Democratic-leaning seat |
| Florida | New map in May 2026 | Up to 4 more Republican-leaning seats |
| Tennessee, Louisiana, Alabama | New maps in May and June 2026 | About 1 more Republican-leaning seat each |
| Missouri | Passed a new map in 2025, but used its 2022 map for 2026 amid a referendum challenge | None for 2026 |
| Virginia | Voters approved a redistricting amendment in April 2026; the state Supreme Court struck it down in May | None for 2026 |
“Intended effect” means what the mapmakers targeted. Whether those seats actually change hands depends on candidates, turnout and the national mood on November 3. If you follow specific races, our guide to tracking the 2026 House races explains how to check which lines your district is using now.
What do the courts allow? #
Partisan gerrymandering: federal courts stay out #
In Rucho v. Common Cause (2019), the Supreme Court held that partisan gerrymandering claims are political questions federal courts can’t decide. Challenges now go to state courts, under state constitutions, or to voters through ballot measures. Utah’s 2025 court-ordered map is an example of the state-court route.
Racial gerrymandering and the Voting Rights Act #
Drawing lines to dilute the votes of racial minorities is still illegal, and Section 2 of the Voting Rights Act has required majority-minority districts in some places. In 2023, Allen v. Milligan upheld that requirement in Alabama. Then on April 29, 2026, in Louisiana v. Callais, the Court ruled 6–3 against Louisiana’s map with a second majority-Black district and made Section 2 vote-dilution claims considerably harder to win. Within weeks, Louisiana, Tennessee, Alabama and Florida adopted new maps. The dissent argued the decision leaves Section 2 with little force; the majority said race-based districting must meet a stricter standard. Expect litigation over these maps to continue after 2026.
Independent commissions #
About a dozen states give line-drawing to commissions rather than legislators, including Arizona, Colorado, Michigan and Washington. California’s commission map was set aside for 2026 through 2030 by Proposition 50. Commission maps tend to produce more competitive districts, though “independent” means different things in different states.
How to see gerrymandering in the numbers #
- Compare the national House vote to the seat split. If one party wins the popular vote by 2 points and ends up in the minority, maps are part of the story.
- Watch the generic ballot. It measures national preference for Congress. With 2026’s new maps, the same generic-ballot margin could translate into a different number of seats than in 2024. Our explainer on the generic ballot covers how analysts convert one to the other.
- Count competitive seats. Fewer Toss-ups after a redraw means the map is doing more of the work.
Election Tracker, our free iPhone app, shows a 30-day average of generic-ballot polls on its House tab, with each poll’s pollster, sample and dates. It also shows read-only Polymarket prices for House control on its Market Sentiment tab. Putting the two side by side is a quick way to see the gap between how people say they’ll vote and how traders think the seats will fall.
Frequently asked questions #
How often are congressional districts redrawn? #
After every census, so the next regular redraw follows the 2030 count. States can legally redraw more often unless their own laws forbid it, which is what happened in 2025 and 2026. Our explainer on why the House is limited to 435 seats covers how apportionment sets each state’s number of seats.
Is partisan gerrymandering illegal? #
Not under federal law, after Rucho v. Common Cause in 2019. Some state constitutions or voter-approved laws ban it, and state courts have struck down maps under those provisions. Racial gerrymandering remains unconstitutional, though Louisiana v. Callais changed how hard it is to prove.
Does gerrymandering affect Senate elections? #
Not directly. Senators run statewide, and state lines can’t be redrawn. Gerrymandering can shape the pool of future Senate candidates and state legislatures, but a Senate race is decided by the whole state’s vote.
Can voters stop gerrymandering? #
In states with citizen ballot initiatives, voters have created independent commissions or anti-gerrymandering rules, as in Michigan and Utah. Voters can also use referendums to challenge new maps, as happened in Missouri. In other states, change has to come from the legislature or state courts.