California’s 4th Congressional District holds its top-two primary on June 2, 2026, under the state’s nonpartisan jungle primary rules that advance the two highest vote-getters regardless of party. Longtime incumbent Democrat Mike Thompson, first elected in 1998 and a moderate Blue Dog member, faces a primary challenge from fellow Democrat Eric Jones, a self-funded venture capitalist and nonprofit executive who has spent heavily on the race. The district, redrawn after Proposition 50, now encompasses Napa and Sonoma wine country plus more rural, conservative areas in Yolo, Yuba, and surrounding counties. Multiple Republican candidates, including Sharon Brown, Mandy Ghusar, and others with limited reported spending, are expected to split that vote. Thompson and Jones have each raised and spent millions, positioning them as the clear frontrunners for the two advancement spots.