Mike Thompson Favored at 96% to Win CA-04 in November

CA-04's Democratic Wave: How a 46-Point Market Surge Exposes What the Primary Already Told Us

California's jungle primary on June 2 delivered exactly the result the Democratic Party needed in the 4th Congressional District: incumbent Mike Thompson advanced to the November ballot alongside Republican Raymond Riehle, while the only credible Democratic challenger, Eric Jones, was eliminated entirely with just 20.9% of the vote. The general election matchup is now set in a district redrawn to favor Democrats, and the prediction market has responded accordingly.

On both Kalshi and Polymarket, the Democratic Party's implied probability of winning CA-04 surged from 50% to 96% over the past three days, a 46-percentage-point correction. That move looks dramatic on a chart. In reality, it is the market catching up to arithmetic that was available the moment primary results posted. Thompson and Riehle combined for 61.4% of the vote. Jones, who outspent the incumbent with roughly $3.2 million raised, is now off the ballot. The only path to a Democratic loss runs through Riehle winning a district that was explicitly redrawn to lean blue. That is not a plausible scenario at 96% implied probability. It is, however, the correct one.

Before explaining why the market is right to land here, it helps to understand the primary results that made November a formality.

Mike Thompson's Top-Two Finish: The CA-04 Primary Result That Made November a Formality

California's top-two primary system sends the two highest vote-getters to the general election regardless of party. In CA-04, that system functioned as a sorting mechanism that eliminated intra-party risk for Democrats while exposing Republican weakness. Thompson received 37,718 votes (37.5%), and Riehle received 24,047 votes (23.9%). Jones, the well-funded Democratic insurgent, pulled 20,997 votes (20.9%) and finished third. Additional Democratic candidates likely absorbed several more percentage points.

The combined Democratic vote share in the primary dwarfed the Republican total. CA-04's redistricted boundaries now include liberal Napa and Yolo counties alongside more conservative Sacramento Valley territory, but the net partisan lean favors Democrats. Thompson has held this seat since 1998 and demonstrated the kind of incumbent resilience that characterized multiple California Democratic primaries this cycle. With Jones eliminated, there is no November spoiler. Thompson faces Riehle in a one-on-one matchup in a district designed for a Democratic win.

The redistricting strategy is part of a broader national effort by California Democrats to counteract Republican map-drawing advantages in other states. CA-04 is among several redrawn districts where the primary results confirmed the intended partisan outcome.

Watch the CA-04 Market Move in Real Time: Democratic Party at 96%

Loading live prices…

The current 96% price is identical on both Kalshi and Polymarket, a zero-point spread that signals strong consensus. When two independent platforms converge on the same implied probability, it typically means the informational inputs are unambiguous. In this case, those inputs are the certified primary results and CA-04's partisan composition.

At 96%, the market is pricing in roughly a 1-in-25 chance that Riehle wins. That residual 4% accounts for extreme tail risks: a Thompson health emergency, a major scandal, or a dramatic shift in the national political environment between now and November 3. None of these are forecasted. The 4% is uncertainty tax, not a real competitive margin.

For context, districts with similar partisan leans and incumbent advantages routinely trade at or above 95% on prediction markets once the primary clears the field. CA-04 is not an outlier. It is a textbook case of a market belatedly adjusting to structural certainty.

The Price History of CA-04: How the Democratic Party's 96% Was Built Over Time

The 50% starting point is the tell. Before June 2, the market was pricing genuine uncertainty, and for good reason. Jones's $3.2 million war chest posed a real threat to Thompson in the primary. If Jones had finished in the top two instead of Thompson, the general election calculus could have shifted: a political newcomer against an established Republican in a district with conservative pockets is a different proposition than a 28-year incumbent in the same race. The market was hedging against the possibility that Democrats would cannibalize their own advantage.

The moment primary returns confirmed Thompson in first and Jones eliminated, the entire risk profile collapsed. The 46-percentage-point move occurred not because new political information emerged after the primary, but because the primary itself was the information event. Markets that sat at 50% before June 2 were pricing a contested intra-party race. Markets at 96% after June 5 are pricing a non-competitive general election. Both prices were rational for their respective information states.

The Case Against 96%: What Would Have to Go Wrong

Any honest assessment of a 96% implied probability must grapple with the downside scenario. For Riehle to win CA-04 in November, several conditions would likely need to converge. First, the national political environment would need to shift decisively toward Republicans in a way that overrides district-level partisanship. Midterm elections can produce wave dynamics, but a swing large enough to flip a redrawn blue-lean district would imply a Republican pickup of 30 or more House seats nationwide.

Second, Thompson would need to lose Democratic base support. At 74 years old (first elected in 1998), age could become a factor if his campaign energy flags between June and November. Riehle, a business owner and entrepreneur, could theoretically run a localized campaign focused on Sacramento Valley economic concerns that peels off moderate voters. Third, turnout differentials in a midterm year could narrow the partisan gap if progressive voters in Napa and Yolo stay home.

These scenarios are not impossible. They are, however, individually unlikely and collectively improbable. The 4% residual probability is a reasonable accounting of these tail risks. Riehle would need a political environment that does not currently exist, combined with a Thompson stumble that has no current evidence. The market at 96% is not overconfident. It is correctly reading a race where the structural outcome was determined on June 2.

Resolution and What to Watch

This market resolves on November 3, 2026. Between now and then, the price is unlikely to move materially unless one of the tail-risk scenarios described above begins to materialize. Traders looking for value in CA-04 will find none at 96%. The informational edge was available to anyone who read the primary returns five days ago. The market has now fully absorbed that edge, and the spread across platforms confirms there is no arbitrage opportunity remaining.

The broader lesson: in California's top-two system, prediction markets on general election outcomes are often mispriced before the primary and correctly priced immediately after. CA-04 is a clean example of that pattern. The Democratic Party's 96% is not a prediction. It is a description of the race that already exists.