California's 4th Congressional District holds its top-two primary on June 2, 2026, with incumbent Mike Thompson seeking his 15th term against Democratic challenger Eric Jones and a field of six Republicans plus one independent. Thompson's long tenure, moderate record, and established fundraising position have positioned him as the clear frontrunner, while Jones has drawn attention through substantial early spending and high-profile endorsements that could consolidate progressive support. The district's heavy Democratic registration and partisan lean make it likely the top two finishers will both be Democrats, leaving Republican candidates with limited paths to advance. No major late developments have shifted the candidate lineup since filing closed in March, leaving voter turnout and final-week messaging as the remaining variables before results determine the general-election matchup.