🗺️ Gerrymandering Simulator — Packing & Cracking Districts

For each district, "wasted votes" are counted for both parties: all votes cast for the losing party, plus any votes the winning party received beyond the minimum needed to win (roughly half the district's votes plus one). The efficiency gap is the difference between the two parties' total wasted votes, divided by the total votes cast, expressed as a percentage. A large efficiency gap indicates one party's votes are being wasted far more than the other's — a signature of packing and cracking.

Legislative seats are awarded per district, not by statewide popular vote, so the geographic arrangement of voters into districts determines outcomes as much as raw vote totals do. A party can win a majority of total votes across all districts yet win a minority of seats if its supporters are inefficiently distributed — concentrated in a few districts (packed) or spread as consistent minorities (cracked) in many others.

The word combines "Gerry," after Massachusetts Governor Elbridge Gerry, and "salamander," describing the shape of an 1812 state senate district he signed into law that critics said resembled the amphibian. The term has since become the standard label for any manipulated electoral map, regardless of which party benefits.

Rules vary widely. Racial gerrymandering that dilutes minority voting power is prohibited under the U.S. Voting Rights Act and constitutional case law. Partisan gerrymandering, however, was ruled a non-justiciable political question by the U.S. Supreme Court in Rucho v. Common Cause (2019), leaving state courts, state constitutions, and independent redistricting commissions as the primary remedies in many jurisdictions.

Several U.S. states, including California, Michigan, and Arizona, have transferred district-drawing authority from state legislatures to independent commissions designed to reduce partisan control. These commissions typically use explicit compactness and competitiveness criteria similar to the "bias = 0" fair-map setting in this simulation, aiming to minimize the packing and cracking that produces large efficiency gaps.

Advances in computing power let map-drawers generate and score thousands of potential district plans per second, testing each for partisan advantage down to the precinct level. This has made gerrymandering far more precise and durable across multiple election cycles than the manual, map-by-hand redistricting of past decades, which is one reason algorithmic and statistical detection tools like the efficiency gap have become important countermeasures.

Researchers use Markov chain Monte Carlo methods to generate large ensembles of valid district maps and compare a proposed map's partisan outcomes against that distribution, flagging statistical outliers as likely gerrymanders. Other active areas include multi-metric fairness scoring beyond the efficiency gap, automated compactness measurement, and open-source redistricting tools that let the public evaluate and propose alternative maps during official redistricting processes.