Incumbent Democratic Representative Mike Thompson holds a commanding position in California's 4th Congressional District top-two primary on June 2, 2026, reflecting his 28-year tenure and the district's consistent Democratic lean. Multiple Republican candidates have filed but remain fragmented with limited resources, reducing their prospects of securing one of the two advancement spots under the state's nonpartisan primary rules. Democratic challenger Eric Jones, backed by substantial fundraising and endorsements including from progressive groups, has emerged as the clearest rival to Thompson, intensifying intra-party competition while preserving the strong likelihood of a Democrat-on-Democrat November matchup. The filing deadline passed in March with no major shifts since, leaving trader consensus focused on these two frontrunners advancing amid stable polling trends and the absence of late-breaking developments.