Google cofounder Sergey Brin spent much of his career on the sidelines of electoral politics. A proposed California wealth tax has changed that, turning the billionaire into one of the state's most powerful political financiers and prompting him to build an organization that can keep shaping state policy long after voters decide on the measure that first 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 the 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 had spent more than $304 million supporting efforts to defeat Proposition 40 and advance Propositions 41 and 42 — direct counters to the wealth tax. Approximately $118.5 million of that went toward opposing Proposition 40, while the remaining $186 million backed Propositions 41 and 42, according to Capital & Main. Those figures do not capture spending in the final stretch before Election Day.

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 proposition receives 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.

The wealthy opposition extends well beyond a single organization. According to a Politico analysis, 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.

Building a Better California has also endorsed Propositions 1, 3, 37, 41, and 42, according to its policy platform. The group separately put $10 million behind Proposition 45>), a 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 mobilization was Proposition 40>), a proposed one-time tax on the wealth of California billionaires set to 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. It 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 (SEIU-UHW), which argued that California needs additional funding to protect its health care system as federal cuts threaten coverage for residents. The measure is also backed by progressive leaders Sen. Bernie Sanders (I-Vt.) and Rep. Ro Khanna (D-Calif.).

Brin's response: a move to Nevada and a $102 million campaign

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 — a timing that placed him outside the pool of California residents the measure would reach as of that date.

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, according to California campaign finance filings. By August, his contributions had reached $102 million — more than five times his opening stake.

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, including a stint as 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." (X post, X post)

Some billionaires just want the wealth tax dead

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 agenda, and former Google CEO Eric Schmidt was among the ultrawealthy backers who took issue with Brin's wider 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 that 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." The comment pointed to a divergence between Schmidt and Brin over the group's money going to other measures, such as Proposition 45.

This story was originally featured on Fortune.com.