Methodology
Scoring districts
For each plan, election returns are assigned to congressional districts and summed. The district score is the Democratic share of the two-party presidential vote. Districts above 50 percent Democratic are counted as Democratic-majority districts. District partisanship categories use the two-party margin, with the party label assigned to the party ahead in the district:
- Toss-up: 0 to 2.5 points
- Lean: 2.5 to 10 points
- Likely: 10 points or more
National environment
The national environment control is measured using the two-party popular vote margin. A D+2 environment means a national Democratic two-party vote share of 51 percent.
Enacted maps
The 2022 and 2024 enacted maps use the congressional plans in effect for those elections. The 2026 enacted map uses new district assignments for states with mid-decade changes. All other states keep their 2024 congressional map.
Simulated maps
The simulated maps are selected from the ALARM congressional simulation ensemble (McCartan et al. 2022). They are selected separately within each state.
For each state, all sampled simulated plans are scored with the 2024 presidential vote. The plans are then ranked by the number of Democratic-majority districts. The atlas keeps five examples from that state distribution:
- the plan with the most Democratic-majority districts
- the plan at the 75th percentile of Democratic-majority districts
- the plan at the median
- the plan at the 25th percentile of Democratic-majority districts
- the plan with the fewest Democratic-majority districts
When multiple plans tie on seat count, ties are broken by the margin in the most competitive district of the relevant party. For plans at or below the median, the tiebreaker favors the plan where the most competitive Republican-held district has the largest margin. For plans at or above the median, it favors the plan where the most competitive Democratic-held district has the largest margin. Within a tied group, the quantile plan is then chosen at the corresponding percentile of that ordering.
The national simulated maps shown in the atlas combine those state-level choices. For example, the median simulated map uses the median selected plan within each state, not one single national simulation draw.
Optimization maps
The Safe Max and Max maps are prototype safe-seat-maximizing and seat-maximizing optimizations generated with the shortburst method (Cannon et al. 2023) using redist (Kenny et al. 2026). They use the multi-district merge-split sampler as the backend for states with at least three congressional districts (Kenny 2026) and standard merge-split for states with two congressional districts (Autry et al. 2021). These maximize the number of seats that each party would win with at least 55% or at least 51%, respectively, under the 2024 presidential vote. Short bursts do not guarantee that they are the global maxima or minima. Further, these are not necessarily the “optimal gerrymanders” in the sense that parties may prefer optimizing different goals, such as protecting particular incumbents or making seats differentially safe based on recent electoral trends.
Partisan fairness
The atlas reports four national partisan fairness metrics computed from district-level presidential vote shares under each plan. Positive values indicate a Republican advantage, and negative values indicate a Democratic advantage.
Partisan bias compares Democratic seats at the selected national two-party vote share with Republican seats under a uniform swing to the mirror-image national vote share, then halves the difference. The efficiency gap compares each party’s wasted votes as a share of all two-party votes. Wasted votes are losing votes plus winning votes beyond the minimum needed to win the district. Dilution asymmetry uses the same wasted-vote idea, but divides each party’s wasted votes by that party’s total votes before taking the difference. Declination compares how steeply each party’s won seats move away from the 50% threshold.
All four metrics update with the selected baseline year and slider for the national environment.