Google cofounder Sergey Brin has committed more than $100 million to oppose California's proposed wealth tax, according to a Friday filing that revealed an additional $20 million contribution to Building a Better California, a political action committee and advocacy group fighting the ballot measure.

Brin's total donations to the organization now stand at $102 million. The PAC also supports pro-business policies and housing and infrastructure affordability initiatives.

Proposition 40, set to appear on California's November ballot, would levy a one-time 5% tax on the state's approximately 200 billionaires. If passed, it would become the first net-worth wealth tax enacted by any U.S. state, a distinction that has drawn national attention at a time when federal wealth tax proposals—most notably from senators Elizabeth Warren and Ron Wyden—have failed to advance in Congress. Under the California measure, 90% of revenue would fund the state's health care program, while the remaining 10% would support education, food assistance, and administrative costs. Brin, whose net worth is estimated at nearly $270 billion, could face a tax bill exceeding $13 billion if the proposition passes.

California, the nation's most populous state, has emerged as a focal point in debates over the so-called K-shaped economy—the widening gap between wealthy and non-wealthy residents. The state boasts a $4 trillion GDP, roughly comparable to that of the United Kingdom, yet 18% of its residents live below the poverty line, the highest rate in the country, driven in part by elevated living costs. The proposition also arrives against a backdrop of recurring state budget shortfalls, including a deficit that exceeded $47 billion in the 2024 fiscal year, which has intensified legislative pressure to identify new revenue sources.

Billionaire Opposition to Proposition 40

The ballot initiative has drawn fierce resistance from several of California's wealthiest figures. Former Google CEO Eric Schmidt and PayPal cofounder Peter Thiel have both donated to groups working to defeat the measure.

Brin, who emigrated from the Soviet Union as a child, drew a direct comparison between the proposal and his socialist upbringing. In an April statement to the New York Times, he said: "I fled socialism with my family in 1979 and know the devastating, oppressive society it created in the Soviet Union. I don't want California to end up in the same place."

California Governor Gavin Newsom has aligned with the tech billionaires in opposing Proposition 40, arguing it would erode the state's tax base and ultimately reduce funding for essential public services. California already imposes the highest top marginal state income tax rate in the nation at 13.3%, a fact frequently cited by critics who argue the state's tax burden is already a driver of outmigration among high earners.

"The fact is it actually will reduce investments in education," Newsom said in a Bloomberg Businessweek interview earlier this year. "It will reduce investments in teachers and librarians, childcare. It will reduce investments in firefighting and police."

Opposition to the measure may be partly self-reinforcing, as billionaires including Brin relocate business interests and personal residences to states without comparable wealth taxes. State records show Brin now lists Nevada as his primary residence. Larry Page, who cofounded Google with Brin, has transferred multiple assets out of California, including Koop, his family office, which was incorporated in Delaware in December 2025. Oceankind, an ocean science nonprofit founded in 2018 by Page's wife, Lucy Southworth, was similarly incorporated in Delaware around the same period. Brin also reportedly purchased a $51 million home near Miami Beach in March.

Would a Wealth Tax Drain California?

Evidence remains unclear as to whether billionaire anxieties about California's economic future are justified. The six billionaires expected to depart the state—Brin, Page, Thiel, car loan magnate Don Hankey, former Uber CEO Travis Kalanick, and director Steven Spielberg—would have collectively generated approximately $27 billion in tax revenue, representing roughly one-fourth of the $100 billion the proposal is projected to raise over five years.

However, billionaires were already contributing comparatively little in California income tax, meaning their departure may have a smaller fiscal impact than opponents suggest. A May working paper from the National Bureau of Economic Research found that California-resident billionaires paid $4.1 billion in income tax last year—approximately 0.2% of their combined $2 trillion net worth. Even if every billionaire left the state, it would take roughly 25 years for lost income tax revenue to offset the projected $100 billion from the wealth tax. A mass exodus of one-quarter of the state's wealthiest residents would take a century to equal that sum.

"The proposed one-off California billionaire tax of 5%, payable over five years, is both small relative to California billionaires' wealth gains and large relative to the taxes they currently pay," the paper's authors wrote.

This story was originally featured on Fortune.com.