California's billionaires have poured an unprecedented $612 million into state elections, with a large share of the money aimed at defeating a proposed wealth tax headed for the November 3 ballot.

The tax, Proposition 40, would impose a one-time levy of 5% on the net worth of billionaires residing in California as of January 1, 2026. Unlike an income tax, which applies to earnings in a given year, the measure would apply directly to wealth that has already been accumulated — an approach that remains rare in state-level taxation in the United States. The scale of the giving marks a record level of political contributions by the state's wealthiest residents in a California election cycle.

Among the most prominent opponents of the measure are Google co-founder Sergey Brin, former Google chief executive Eric Schmidt, and veteran venture capitalist L. John Doerr — all of them prominent figures in California's technology industry. California's ballot-initiative system allows proposed laws to be decided directly by voters, and this cycle's ballot features competing measures with direct consequences for Proposition 40.

The opposition effort is also supporting Propositions 41 and 42, either of which could potentially nullify Proposition 40 if it receives more votes. Under that scenario, the fate of the wealth tax would rest on the relative tallies of the competing initiatives rather than on Proposition 40 alone.

Market activity suggests the heavy financial opposition is shaping expectations for the vote. Prediction markets, where participants trade on the outcomes of real-world events, convert collective expectations into implied probabilities. Market pricing indicates that the scale of billionaire contributions appears to decrease the likelihood of Proposition 40 passing. Current market odds imply a 26.5% probability that the wealth tax will pass — a slight increase from previous days despite the wave of opposition spending. The organized financial backing against the measure also points to a coordinated effort to influence the election's outcome.

The coming days leading up to the November 3 vote are expected to be pivotal. Shifts in polling data or new endorsements could sway public opinion, and the actions of key political figures — including Governor Gavin Newsom and other influential candidates — may play a decisive role in the result. The outcomes of Propositions 41 and 42 will also be critical, since either holds the potential to counteract Proposition 40's impact.

As election day approaches, continued financial contributions from both sides will further clarify market expectations for one of the most closely watched measures on California's statewide ballot.