Campaign filings show opposition to California Proposition 40 neared $425m

Sergey Brin gave at least $132m, the opposition’s biggest donation by far

Campaign finance filings show the opposition to California Proposition 40 mounted a nearly $425m campaign, making it the most expensive opposition campaign in state history. Google co-founder Sergey Brin was the opposition’s biggest donor by far, contributing at least $132m to fight the tax. In the first two weeks of October, opposition spending surpassed $2.5m a day, according to the most recent campaign finance filings. Voters rejected the measure.

The Guardian described the ballot measure campaign as contentious and reported that California billionaires won it, defeating the proposed one-time tax on their wealth. The campaign over Proposition 40 was the most expensive in state history and drew back-and-forth dealing between tech billionaires and Democratic leaders, including Gavin Newsom, the governor, who steadfastly opposed the measure.

Donors to the opposition campaign included executives from Google, Palantir, DoorDash, Stripe and Reddit, along with several prominent venture capitalists.

The opposition put two counter-measures on the ballot, Propositions 41 and 42. Proposition 41 directly targeted the billionaire tax, while Proposition 42 was a general proposal on limiting certain new taxes and would ban any taxation on billionaire wealth. Under that strategy, if either measure received more yes votes than Proposition 40, it would stop the billionaire tax from taking effect, even if Proposition 40 passed. Supporters of the billionaire tax called the two measures “poison pills.”

A Bloomberg/Getty Images photograph shows supporters holding signs advocating for the Billionaire Tax Now coalition in Los Angeles in April.

Proposition 40 proposed a one-time 5% tax on all individuals in California with a net worth of more than $1bn as of January 1, 2026. Sponsored by the healthcare union SEIU-United Healthcare Workers West, the measure would have directed proceeds from the tax to healthcare programs, along with some education and food assistance. Supporters cast the measure as a solution to drastic healthcare cuts from Trump’s One Big Beautiful Bill Act.

California’s major unions backed Proposition 40, as did Vermont Sen. Bernie Sanders and California Rep. Ro Khanna. Both the Democratic and Republican candidates for governor opposed the measure, as did organizations including Planned Parenthood Affiliates of California, a lobbying branch of the healthcare group. They pointed to its structure as a one-off tax that would only fund certain healthcare programs.

Opponents led by billionaires said the billionaire tax was “an everyone tax.” The Guardian identified California’s billionaires as Proposition 40’s most vocal critics.

The Guardian said the rejection could hamper nationwide momentum on demands that billionaires pay their fair share and for states enacting new laws to tax the ultra-wealthy.