Incumbent Mike Thompson (D) and Eric Jones (D) advanced from the top-two primary for California's 4th Congressional District on June 2, 2026. Six Republicans and one independent candidate also ran. As of March 2026, Thompson (D) and Jones (D) led in fundraising and local media attention.

Local political observers described the primary in terms of the ideological and generational contrast between Thompson and Jones. The San Francisco Chronicle's Joe Garofali described the primary as "another example of a younger, well-funded Bay Area Democrat taking on an entrenched candidate as the Democratic Party grapples with how to win back voters," referring to the party's losses at the national level in the 2024 elections. The Sonoma Index-Tribune's Ruchi Shahagadkar said Thompson "has championed the region’s wine industry, fought for federal tax relief for wildfire survivors and served as the House Democrats’ point person on initiatives combating gun violence." Politico's Jeremy B. White said Jones is "hoping to exploit a rapidly shifting media landscape that makes it easier for lesser-known candidates to break through, as Zohran Mamdani did in New York’s mayoral primary with viral campaign videos."

Thompson was first elected to the House in 1998. He earlier served eight years in the California Senate. Thompson said he was running because "[w]ith the outcome of the last election and what’s happened since then with this (Donald Trump) administration and the Republican majority, it’s absolutely imperative that I continue the work I’ve started," referring to his position on the House Ways and Means Committee. The Democratic Party of California endorsed Thompson.

Jones was a former executive at an investment firm and the founder of the American Dream Institute, a group describing itself as "a first-of-its-kind digital engagement engine dedicated to helping the Democratic Party rebuild trust with young working Americans." Jones said he was running "to restore the American Dream for every family — not just the wealthy few." Our Revolution, an organization that advocated for the policies of Sen. Bernie Sanders (I-Vt.), endorsed Jones.

Also running in the primary were Sharon Brown (R), Mandy Ghusar (R), Jimih Jones (R), L. John MacKenzie (R), Raymond Riehle (R), Chuck Uribe (R), and Thomas Roach (I).

In a top-two primary, all candidates running for a given office appear on the same ballot. The top two finishers—regardless of partisan affiliation—advance to the general election. One Democrat and one Republican had, as of the 2026 primary, advanced from every top-two primary in the 4th District since 2016. Citing California State University, Sacramento, professor Wesley Hussey, The Sacramento Bee's Jake Goodrick said, "A Republican candidate often advances in a top-two primary, even in a heavily Democratic district...but without a stand-out Republican to back, a scenario in which the four Republicans split votes could favor both Thompson and Jones advancing."

The primary took place in the context of redistricting in California that changed the 4th District's boundaries from those used in 2024. Inside Elections' Nathaniel Rakich said the 2026 version of the 4th District favored Democrats overall but did so by a narrower margin than the district lines in use in 2024. As of June 2026, major election forecasters rated the general election Solid/Safe Democratic.