California’s new congressional map, enacted after voters approved Proposition 50 in November 2025, is the dominant driver of trader expectations. The legislatively drawn boundaries shifted several formerly Republican-leaning districts in the Central Valley and San Diego area toward stronger Democratic performance, with analysts projecting net gains of three to five seats relative to the 43 Democratic seats held after 2024. June 2026 primaries confirmed Democratic nominees in the targeted districts, including those formerly held by Republicans such as Darrell Issa, while leaving only a handful of competitive Republican strongholds. National midterm dynamics favoring the opposition party have further reinforced the outlook for Democrats reaching 47–48 seats, with the market’s heavy weighting toward 48 reflecting the cumulative effect of these map changes and limited remaining uncertainty in the final months before the November election.