Same voters, 16 maps: Democrats can count on as few as 102 seats or as many as 297, and anywhere from 11 to 223 are up for grabs.

The building blocks

Every alternative district is assembled from whole 2020 Census voting districts — 187,560of them, block groups in California, Hawaii, Oregon and West Virginia — with their 2020 population and voting-age population by race, as packaged by Dave’s Redistricting. Seats follow the 2020 apportionment, the same 435 on the ballot this year.

Partisanship is the 2024 presidential vote, precinct by precinct. In the nine multi-seat states whose 2024 precinct returns are not yet in that package (Arkansas, Connecticut, Idaho, Maine, Michigan, New Jersey, Oklahoma, Oregon and Pennsylvania), each 2020 precinct result is carried forward by its own county’s actual 2020-to-2024 swing.

The rules every map obeys

Every district is contiguous — each precinct reachable from every other through shared borders, water crossings only where islands require them — and within 1% of its state’s ideal population. Across all 6,525 drawn districts the largest deviation is 0.99%, and half the states’ maps stay within 0.09%. A build step re-checks both rules for every district independently and refuses to publish a map that fails either.

Whole precincts cannot balance districts to the single person, as enacted maps do by splitting census blocks. Anyone adopting one of these maps would finish that last step at block level.

How a computer draws a map

Each map starts from a random legal map and is improved by two kinds of move. ReCom merges two neighboring districts and splits them again along a random spanning tree, exploring very different maps quickly; runs of ten such moves keep the best map found (“short bursts”). Simulated annealing then moves single precincts across lines to finish the job. Searches also start from the enacted map, and the better result wins.

No goal may buy its result with tentacles: the regular gerrymanders and the proportional, competitive and minority maps cannot cut more than 1.3 times the precinct borders of a random legal map, and the fair-and-tidy, tidy competitive and light gerrymander maps no more than a typical map’s borders and county splits. The clean-lines maps are scored on exactly those. Nothing was tuned state by state or drawn by hand.

An even year, by default

Every map is drawn for, and shown in, a year when the national popular vote is even — the way FiveThirtyEight’s 2018 Atlas read districts by their lean relative to the nation. Trump carried the 2024 two-party vote by 1.6 points in these precincts, so an even year moves every district 1.6 points toward the Democrats. A map that splits seats fairly should then come out near 218 to 217. The slider moves the national vote, with presets for the 2024 result and our 2026 forecast.

A district’s chance comes from its lean through one curve fitted to our own House forecast (edition of 2026-09-19), whose spread is 7.2points; at the forecast’s environment the curve reproduces its 229expected Democratic seats on today’s map within a seat (229.8). It is an average relationship, not a race forecast: it knows nothing of candidates, incumbents or local polls — the full forecast for today's map is 231 Democratic seats as of 2026-09-26.

The no-goal test

For each of the 44 multi-seat states, a ReCom chain drew 2,000 maps under the same rules and no goal at all; 23,202 of them, spaced through the chain, form the comparison. Such ensembles are how courts and scholars now judge whether an enacted map is an outlier. They describe what neutral rules tend to produce — they are not a claim that any single neutral map is the fair one.

How strong is a gerrymander?

Each party’s gerrymander comes in three strengths. Maximumis the most aggressive map the search can draw: contiguous and equal in population, with boundaries up to twice as long as a typical no-goal map’s and counties split at will. Regular is the most seats it finds within ordinary shape limits — what a determined legislature draws. Light tilts a quarter as far as the maximum from the middle of the no-goal maps, with boundaries no longer and no more split counties than a typical neutral map: a gerrymander that looks like any other map.

Incumbent protection is the bipartisan deal: each state keeps today’s expected split, and every seat is made as safe as the lines allow.

Fair maps and clean lines

Match the electorate makes each state’s expected seats track its own vote; fair and tidydoes the same inside a typical map’s shape and county limits, the map a good commission might draw. Maximum competition makes as many districts as it can where both parties have at least a 1-in-4 chance; its tidy version keeps ordinary shapes. Most compact cuts the fewest precinct borders, keep counties whole splits the fewest counties, and cleanest lines does both: the most compact map that splits no more counties than the county map.

What is new since 2018

FiveThirtyEight’s original Atlas drew its maps by hand on 2010 geography and scored them with the Cook PVI. This edition is drawn by algorithm on 2020 geography, scored with 2024 returns through a curve fitted to our 2026 forecast, lets the reader move the national vote, grades gerrymanders by strength, runs an arms race by who actually controls each state’s lines, adds fair and clean-lines maps, measures the efficiency gap and median seat, tests every enacted map against thousands of goal-free maps, and publishes every map as a shapefile, GeoJSON and exact precinct assignment.