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2026 House Structural Baseline: Open Seats, Redistricting in Effect, and Midterm Swing by Seat Category

The structural prior for the 2026 House map, fixed rather than tracked, in three components joined onto one district identifier. First, an exhaustive open-seat list: every district whose incumbent has retired, resigned, died, lost a primary, or is seeking another office, carrying the vacancy cause, the date it became known, the prior-cycle margin, and the district's presidential lean measured on the lines actually in effect rather than the lines the result was recorded under. Second, every redistricting change operative for 2026 — the enacted or court-ordered map, its effective date, the litigation posture that could still move it, and a district-level crosswalk from old numbering to new with the partisan-lean delta, because a district whose number is unchanged is frequently not the same district. Third, historical midterm swing decomposed by seat category: open versus incumbent-held, freshman versus senior incumbent, and by presidential-lean band, measured across postwar midterms with the president's-party split reported separately. The decomposition is the reason to buy it — the open-seat penalty and the incumbency advantage move in opposite directions, and the headline average seat loss conceals both. Together these give the baseline a forecast starts from before any polling enters. Method: candidate filings and retirement announcements, enacted maps and court orders with their effective dates, and district-level presidential and congressional returns are reprojected onto one consistent district identifier and joined into four linked tables.