California's 4th Congressional District held its nonpartisan primary on June 2, 2026, under the state's top-two advancement rule, where the two highest vote recipients proceed to the November general election regardless of party affiliation. Incumbent Democrat Mike Thompson led the field, followed by Republican Ray Riehle, with another Democrat, Eric Jones, placing third in partial counts. Thompson's long tenure since 1999 and established fundraising positioned him strongly, while Riehle's performance reflected Republican efforts to consolidate support in the redrawn district. No major late developments altered the field after filing closed in March, and the results align with the district's Democratic lean in recent cycles. Traders monitoring the market focus on confirmation of final certified totals and any recounts in close races among lower-polling candidates.