But a proposed California wealth tax has turned the billionaire into one of the state’s most powerful political financiers, prompting him to build an organization that can continue shaping state policy long after voters decide on the measure that initially mobilized him.

Brin has contributed $102 million to Building a Better California, a political organization that has become the central vehicle for Silicon Valley billionaires seeking to influence California’s political direction. The group has raised hundreds of millions of dollars from wealthy tech executives, venture capitalists, and business leaders—channeling money into ballot measures that extend beyond the immediate fight against Proposition 40.

Two measures written to kill the tax

Through Oct. 20, Building a Better California has spent more than $304 million supporting efforts to defeat Proposition 40 and advance Propositions 41 and 42—direct counters to the wealth tax. Of that amount, approximately $118.5 million went toward opposing Proposition 40, while the other $186 million supported Propositions 41 and 42, according to Capital & Main. 

Proposition 41 would bar the state from exempting new taxes from its spending limit and require state audits of programs funded by new special taxes, making it harder for the state to use funds raised by the billionaire tax. Proposition 42 would outlaw retroactive taxation, directly countering Proposition 40. The strategy gives the ultrawealthy opponents multiple opportunities to defeat the measure: If either one gets more “yes” votes than Proposition 40, the Legislative Analyst’s Office says, the wealth tax could be stopped even if a majority of voters approve it.

And according to an analysis by Politico, more than 70 current and former California billionaires had directed a record $612 million into state elections during the 2026 cycle. Nearly half of that money went toward groups working to defeat Proposition 40, and at least 30 billionaires had contributed more than $293 million specifically to the wealth tax fight, according to the analysis.

Building a Better California also endorsed Propositions 1, 3, 37, 41, and 42, according to its policy platform. It also put $10 million behind Proposition 45, a separate measure addressing California’s environmental review process, and $6 million behind Proposition 37, which would create a $25 billion homebuyer loan fund, according to Capital & Main.

The ballot measure that awakened the billionaire beast

The immediate catalyst for the ultrawealthy movement was Proposition 40, a proposed one-time tax on the wealth of California billionaires that will appear on the Nov. 3 ballot.

The measure would impose a 5% tax on the assets of individuals and certain trusts worth more than $1 billion who were California residents on Jan. 1, 2026. The proposal is designed to raise money to offset federal health care funding cuts, with 90% of the proceeds allocated to health care and the remaining 10% directed toward education and food assistance. According to the Legislative Analyst’s Office, the state probably would collect tens of billions of dollars from the tax, spread across several years.

The proposal was sponsored by the Service Employees International Union-United Healthcare Workers West, which argued that California needs additional funding to protect its health care system as federal cuts threaten coverage for residents. The ballot measure is also backed by progressive leaders Sen. Bernie Sanders (I-Vt.) and Rep. Ro Khanna (D-Calif.).

For Brin, whose fortune is estimated at $231 billion by Bloomberg, the potential financial exposure is substantial—and he has been particularly vocal about his opposition. In an interview with the New York Times in April, he compared the proposal to the socialism he experienced growing up in the Soviet Union—saying he did not want California to end up in the same place. He also relocated to Nevada in December, ahead of the Jan. 1 cutoff date that determines which billionaires would be subject to the proposed tax.

But rather than leave California’s political fight behind, Brin organized a coordinated campaign to prevent the measure from becoming law.

Sergey Brin did not immediately respond to a request for comment from Fortune.

In January, Brin established Building a Better California—contributing an initial $20 million. By August, his contributions had reached $102 million, according to campaign finance filings.

The organization has since become a principal coordinating point for wealthy tech executives. Other major contributors include Kleiner Perkins chairman John Doerr, former Google CEO Eric Schmidt, Stripe CEO Patrick Collison, and former Sequoia Capital partner Michael Moritz.

Even Mark Cuban, a self-identified “libertarian at heart” and a billionaire closely affiliated with Democrats in recent years—such as being a high-profile surrogate for Kamala Harris in the 2024 presidential race—has found himself on the side of his fellow wealthy compatriots. Cuban got into a public feud with Khanna on X over the proposal, eventually calling the politician “insane.”

“This is the biggest f*** you in the history of entrepreneurship,” Cuban wrote on X. “Ever.”

Some billionaires just want the wealth tax dead

But not everyone shares Brin’s broad political goal. His political organization has seen pushback from at least one major donor over how broadly to expand its political agenda. Former Google CEO Eric Schmidt was one of the ultrawealthy sponsors that had issues with Brin’s broader focus on politics. 

The New York Post initially reported in August that Schmidt had grown frustrated with the anti-wealth-tax coalition led by Brin after the group failed to prevent California Democrats from endorsing the measure.

But a close advisor to Schmidt told Fortune in August the former Google CEO was not “irate” about the endorsement outcome and that Schmidt’s “only current involvement in politics is his contributions to defeating the billionaires tax”—meaning Schmidt and Brin diverged over the group’s money going to other measures, such as Proposition 45.