Every decade, after the census, the United States redraws roughly seven thousand legislative districts. The work is technical, largely conducted out of public view, and capable of determining which party controls a legislature for the following ten years without a single voter changing their mind.
The rules that constrain map drawing
Federal law imposes a few firm requirements. Congressional districts within a state must be nearly equal in population under the one-person, one-vote doctrine, with legislative districts allowed slightly more deviation. The Voting Rights Act prohibits maps that dilute minority voting strength and, in certain circumstances, requires districts where a minority group can elect its preferred candidate.
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Beyond that, criteria vary by state: contiguity, compactness, respect for county and municipal boundaries, preservation of communities of interest, and in some states explicit prohibitions on partisan advantage. Current state-by-state rules and litigation status are tracked by the All About Redistricting project at Loyola Law School.
Packing and cracking
Partisan manipulation uses two complementary techniques. Packing concentrates opposition voters into a few districts they win overwhelmingly, wasting their surplus votes. Cracking splits opposition concentrations across several districts so they fall just short in each. Applied together, a party can convert a modest statewide vote share into a durable seat majority.
Modern software makes this precise. Block-level demographic and past-election data, combined with algorithmic optimization, allows drafters to evaluate thousands of candidate maps against partisan performance targets while satisfying formal legal criteria.
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Measuring the distortion
Because a map can be simultaneously compact, contiguous and heavily skewed, scholars developed quantitative tests. The efficiency gap compares wasted votes between parties. Mean-median comparisons examine the difference between a party’s average and median district vote share. Ensemble analysis generates large numbers of computer-drawn maps satisfying the state’s stated criteria and asks where the enacted map falls in that distribution.
An enacted plan that sits at the extreme tail of thousands of neutral simulations is difficult to explain as coincidence, and this method has featured prominently in recent litigation. Methodological work in this area is published by groups including the Metric Geometry and Gerrymandering Group.
What courts will and will not review
The legal landscape shifted significantly when the Supreme Court held that partisan gerrymandering claims present political questions beyond the reach of federal courts. Racial gerrymandering and Voting Rights Act claims remain justiciable in federal court, and the line between racial and partisan motive is often contested because the two correlate closely in many states.
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State courts remain available. Several state supreme courts have struck down maps under state constitutional provisions guaranteeing free and equal elections, and some state constitutions now include explicit fairness standards adopted by ballot initiative.
Independent commissions
A growing number of states assign map drawing to commissions rather than legislatures. Designs differ: some are fully independent with citizen members selected through a screening process, others are bipartisan panels with tie-breaking mechanisms, and some serve only an advisory role.
Evidence on outcomes is mixed but generally favorable. Commission maps tend to be more competitive and less skewed than legislature-drawn maps in the same states, though commissions can deadlock, and the selection process itself becomes a target for influence. Comparative assessments of commission performance are published by the National Conference of State Legislatures.
The competitiveness problem
Gerrymandering is not the only reason few districts are competitive. Geographic sorting means voters increasingly live near others who share their politics, so even neutral maps produce many safe seats. Primaries then become the decisive contest, pulling nominees toward their party’s base.
That interaction is why redistricting reform alone rarely restores competition. Proposals that pair fair maps with changes to primary structure, such as top-two or ranked-choice systems, aim at the combined problem. Whether those changes deliver the intended moderation remains an open empirical question.
