In California's 4th Congressional District, incumbent Democrat Mike Thompson seeks his 15th term in the June 2, 2026, top-two primary against Democratic challenger Eric Jones, who has raised substantial funds and secured endorsements from progressive groups. The district's strong Democratic lean and redrawn boundaries after Proposition 50 favor Thompson's moderate record and long tenure, while Jones focuses on mobilizing younger voters. A crowded field of six Republicans risks splitting the minority-party vote, which could allow both leading Democrats to advance under the state's nonpartisan primary rules. No major late developments have shifted candidate positioning since the March filing deadline, leaving fundraising totals and local turnout efforts as the primary factors traders monitor ahead of election day.