California voters are preparing to decide on Proposition 40, a ballot initiative that would impose a one-time, 5% tax on the net worth of billionaires residing in the state as of January 1, 2026. The measure, which aims to offset federal cuts to Medicaid, has sparked a fierce debate among economists regarding its potential economic impact. While proponents argue the tax could generate significant revenue to address healthcare shortfalls, critics warn it may trigger an exodus of the wealthy, ultimately costing the state billions in lost income tax revenue.
The proposal marks a significant shift from California's historic tax policies, which have leaned toward lower taxes on wealth since the passage of Proposition 13 in 1978. If passed, it would establish the first wealth tax of its kind in the United States. The outcome of this state-level experiment is being watched closely by policymakers nationwide, as it could serve as a model for potential federal wealth tax legislation.
What the Left Is Saying
Progressive economists and policymakers view Proposition 40 as a necessary correction to a tax system that allows the ultra-wealthy to accumulate vast fortunes while paying minimal income taxes. Gabriel Zucman, a UC Berkeley economist who advised Senator Bernie Sanders and former Senator Elizabeth Warren on national wealth tax proposals, argues that California's billionaire wealth has grown by 144% between 2023 and 2025, largely driven by the AI boom. Zucman, along with Jasper Boll and Emmanuel Saez, estimates the tax could raise approximately $100 billion.
Supporters contend that the tax is designed to prevent avoidance by targeting business holdings and using a retroactive residency cutoff. They argue that the strong economic ecosystem in Silicon Valley will keep billionaires in the state despite the tax. Mark Zuckerberg, for instance, reportedly negotiated a deal to sell non-voting shares to cover his potential tax bill while retaining control of Meta, agreeing to remain in California for eight years. Nvidia CEO Jensen Huang stated he was 'perfectly fine' with paying the tax, noting that his family chose to live in Silicon Valley.
Zucman argues that past failures of wealth taxes in countries like France were due to loopholes that Prop 40 closes, such as exemptions for active business assets. He points to Norway, which has maintained a wealth tax since 1892, as evidence that such taxes can be effective without causing significant capital flight. Zucman views the California measure as a potential catalyst for a broader international movement to tax the superrich.
What the Right Is Saying
Conservative economists and business leaders argue that Proposition 40 will harm California's economy by driving away high-income earners. Joshua Rauh, a Stanford economist and senior fellow at the Hoover Institution, co-authored an analysis with Benjamin Jaros that projects the tax could result in a net loss of $39 billion for the state. Rauh argues that the projected revenue of $100 billion is overly optimistic because it does not adequately account for billionaires leaving the state to avoid the tax.
Rauh notes that at least seven prominent billionaires, including Google co-founders Larry Page and Sergey Brin, Peter Thiel, and Steven Spielberg, have already left California prior to the January 1, 2026, residency cutoff. He estimates their departures alone reduced potential tax revenue by approximately $30 billion. Rauh further contends that the legal uncertainty surrounding the tax, including challenges to its retroactive nature and its valuation of privately held companies, could lead to court-ordered reductions in revenue.
Critics also argue that the tax threatens the startup ecosystem by imposing large liabilities on founders before they have cashed out their equity. Rauh states that agglomeration effects, which keep tech companies clustered in the Bay Area, are fragile and that investors will pull capital from California if the wealth tax passes. He argues that the state trades future income tax revenue for a one-time payout, a trade-off that leaves the state worse off in the long run.
What the Numbers Show
According to analysis by Zucman, Boll, and Saez, California billionaires' wealth reached $2.3 trillion by mid-2026. This figure is comparable to the total annual income reported by all Californians on their tax returns. The economists estimate that a 5% tax on this wealth would generate about $100 billion, after adjusting for a 10% reduction to account for potential avoidance and evasion.
In contrast, Rauh and Jaros estimate that after accounting for billionaire departures and legal challenges, the tax would raise only around $30 billion. They further calculate that the state would lose additional revenue as billionaires move their income tax bases out of California. Their model assumes these income tax losses continue indefinitely, leading to a net projected loss of $39 billion.
The debate has been financially intense. According to a report in The New York Times, billionaires have contributed $333 million to a group opposing Proposition 40. The Bay Area Council Economic Institute reports that in 2025, Bay Area companies attracted three-quarters of all U.S. AI funding and more than half of the worldwide total, highlighting the region's economic dominance.
The Bottom Line
The passage of Proposition 40 will serve as a high-stakes test of whether a state-level wealth tax can generate significant revenue without causing capital flight. If the tax passes and raises the projected $100 billion, it could embolden other states and the federal government to pursue similar measures. If it results in the projected $39 billion loss, it may reinforce arguments against wealth taxes as economically detrimental.
Voters will decide not only on the specific mechanics of the tax but also on the broader question of how to address rising inequality and fund public services. The legal challenges that will likely follow the vote will further shape the final outcome and its national implications.