California's Proposition 40: The $100 Billion Billionaire Tax Battle, Explained

California voters will decide on November 3, 2026, whether to enact the nation's first statewide wealth tax. Proposition 40, officially the 2026 Billionaire Tax Act, is a combined initiated constitutional amendment and statute that would impose a one-time 5% tax on the net worth of California residents whose worldwide net worth exceeded $1 billion on January 1, 2026 . Roughly 200 individuals would be affected . The tax is payable in annual installments of 1% over five years (with a small deferral charge), and the revenue — estimated at roughly $100 billion — is intended to backfill federal funding cuts to healthcare, nutrition, and education programs .

How Proposition 40 Works

The tax applies to California residents as of January 1, 2026, based on their worldwide net worth as of December 31, 2026 . Real estate directly held, pensions, and retirement accounts are generally excluded from the calculation, though real estate owned through a business is taxable . Taxpayers can spread payments over five years but must pay more to do so . Of the revenue raised, 90% would flow to a Billionaire Tax Health Account and other dedicated funds for healthcare, food assistance, and education .

Proposition 40 is championed by SEIU-United Healthcare Workers West, California's largest health workers union, as a direct response to federal funding cuts enacted under the 2025 "One Big Beautiful Bill Act" .

Sergey Brin: The $102 Million Opposition

Google co-founder Sergey Brin has emerged as the single largest financial backer of the opposition. As of mid-August 2026, Brin has poured $102 million into Building a Better California, a political nonprofit that funds ballot measures aimed at blocking or undermining Prop. 40 . His contributions have been used to blanket the state with ads and commercials condemning the wealth tax . For context, Brin's net worth is estimated at roughly $267 billion, so his personal cost of the tax would be about $13.3 billion if it passes .

Brin's spending is widely reported as the most expensive single-issue campaign contribution in California history . The organization he funds, Building a Better California, has also bankrolled two competing ballot measures — Propositions 41 and 42 — that could nullify Prop. 40 if they receive more votes .

Where Other Tech Billionaires Stand

Jensen Huang: 'I'm Perfectly Fine With It'

Nvidia CEO Jensen Huang has taken the most accommodating public stance among California's billionaires. In January 2026, Huang told Bloomberg Television: "We chose to live in Silicon Valley and whatever taxes I guess they would like to apply, so be it. I'm perfectly fine with it" . Bloomberg estimated Huang would face a tax bill of roughly $7.75 billion if Prop. 40 passes . At a Stanford Graduate School of Business event in April 2026, Huang urged other billionaires not to leave, saying, "I say to everybody, 'Move to California, don't leave.' It's the highest taxes in the world, but it's OK" .

Mark Zuckerberg and Larry Page: Relocation Signals

Available reporting describes a broader pattern of tech billionaires taking steps to distance themselves from California in response to the measure. Mark Zuckerberg reportedly purchased a $170 million mansion near Miami in 2026 . Google co-founder Larry Page has also made moves to leave the state . However, no direct campaign spending by Zuckerberg or Page against Prop. 40 has been cited in the available sources . Their actions are best characterized as personal relocation decisions rather than active political opposition.

Governor Newsom vs. His Own Party

Gavin Newsom has been a vocal opponent of Proposition 40. In January 2026 he vowed to stop the measure, arguing it has already caused billionaires to leave the state and take their tax revenue with them . He has also pitched a federal-level higher tax on the wealthy as an alternative, calling for a "true minimum tax on billionaires and those with a net worth of $100 million" .

Newsom's opposition puts him strikingly at odds with his own party. The California Democratic Party's executive board voted in August 2026 to endorse Prop. 40, breaking with Newsom and gubernatorial candidate Xavier Becerra . The endorsement passed narrowly after earlier votes to endorse and to remain neutral both failed . Newsom has warned that the tax could destabilize California's economy, while union and party backers say the revenue is needed to protect safety-net programs from federal cuts .

Legal Challenges Ahead

Even if voters approve Proposition 40, it faces significant legal hurdles. The Tax Foundation has identified numerous constitutional challenges, any of which could invalidate the tax in whole or in part . The measure's retroactive application, its definition of residency, and whether a state wealth tax violates the U.S. Constitution's uniformity clause are all expected to be tested in court .

What Happens Next

California's Secretary of State certified Proposition 40 for the November 3, 2026 ballot on June 25, 2026 . If it passes, the tax would apply retroactively to all California residents as of January 1, 2026, with payments beginning in 2027 . Propositions 41 and 42, funded in part by Brin's organization, could override Prop. 40 if they receive more votes . The outcome of this election will determine whether California becomes the first state in the nation to directly tax billionaire wealth — and whether billionaires respond by staying, paying, or leaving.