Incumbent Democrat Mike Thompson leads early returns from California's June 2, 2026 top-two primary in the 4th congressional district, which covers Napa, portions of Sacramento, Yolo, and surrounding counties. Thompson, first elected in 1998 and a senior member of the House Ways and Means Committee, received roughly 37 percent of the vote, ahead of Republican Ray Riehle and fellow Democrat Eric Jones. Under California's system the two highest vote-getters advance to the November general election regardless of party. Trader pricing on primary-advancer contracts aligns with these partial results, reflecting Thompson's long incumbency, fundraising edge, and limited organized opposition in a district that has favored Democrats in recent cycles. No further primary events remain before certification.