California's 4th congressional district primary on June 2, 2026, operates under the state's top-two system, where the highest vote-getters advance regardless of party. Longtime incumbent Mike Thompson, a moderate Democrat, faces his strongest primary challenge in years from Eric Jones, a well-funded Democratic contender with experience in venture capital and voter mobilization efforts. Redistricting under Proposition 50 added rural and traditionally conservative counties, altering the electorate. Multiple Republican candidates are also on the ballot, which could fragment opposition votes in the nonpartisan contest. Filing deadlines have passed, leaving little time for additional developments before voters decide the two nominees.