California's Billionaire Tax

Heads to Voters Amid Fifth Amendment Questions

Proposition 40 would impose a one-time 5 percent levy on the wealth of roughly 200 residents. Legal scholars say the Takings Clause is not its weakest link—but half a dozen other constitutional doctrines might be.

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BLUF (Bottom Line Up Front)

California's Proposition 40, the "2026 Billionaire Tax Act," will appear on the November 3, 2026, statewide ballot. It would impose a one-time 5 percent excise tax on the worldwide net worth of roughly 200 California residents worth more than $1 billion, measured as of Dec. 31, 2026, and applied retroactively to anyone who was a state resident as of Jan. 1, 2026. Proponents estimate it would raise about $100 billion over five years for Medi-Cal, food assistance, and education; independent researchers at the Hoover Institution put net revenue far lower once behavioral and income-tax effects are counted. Legal analysts across the political spectrum agree the measure will be challenged in court, but most say the Fifth Amendment's Takings Clause is a secondary and historically weak argument against tax statutes—the stronger challenges rest on retroactivity and due process, the dormant Commerce Clause, California's own 0.4 percent cap on intangible-property taxation, and a possible bill-of-attainder claim. The measure's drafters built in an expedited path directly to the California Supreme Court, meaning a ruling could arrive before the first payments are due in April 2027.

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What Proposition 40 Actually Does

The initiative, filed by the labor union SEIU-United Healthcare Workers West and drafted by law professors Brian Galle (UC Berkeley), David Gamage (University of Missouri), and Darien Shanske (UC Davis), along with economist Emmanuel Saez (UC Berkeley), would add a new Section 37 to Article XIII of the California Constitution. It authorizes a one-time 5 percent tax on the net worth of "applicable individuals"—state residents as of the Jan. 1, 2026, "tax obligation date"—with net worth exceeding $1 billion. The tax would be assessed against wealth as of Dec. 31, 2026, and could be paid in a lump sum or in five annual installments carrying a 7.5 percent nondeductible deferral charge. Directly held real estate is excluded, though real estate owned through a business entity is not.

The California secretary of state certified the measure for the ballot on June 17, 2026, after proponents submitted more than 1.6 million signatures—nearly double the 874,641 required. A last-minute attempt by SEIU-UHW to trade the 5 percent initiative for a smaller, legislature-passed 2 percent levy was rejected by Gov. Gavin Newsom before the June 25 withdrawal deadline, and the measure proceeded to the ballot as originally drafted.

Because California's Constitution already caps taxes on intangible personal property—stocks, bonds, and similar assets—at 0.4 percent, the drafters concluded an ordinary statute could not authorize a 5 percent levy on the same property; hence the initiative is written as a constitutional amendment rather than a simple law.

The Takings Clause Argument: Real, But Narrow

Critics, including opinion writers aligned with free-market think tanks, have called the measure a "prima facie" taking of private property without compensation. The doctrinal reality is more limited. The Fifth Amendment's Takings Clause forbids government from taking private property for public use "without just compensation," but courts have long treated the payment of taxes as categorically different from a taking. The National Taxpayers Union Foundation's legal analysis of the initiative notes plainly that "it is rare for a tax to potentially implicate the Takings Clause of the Fifth Amendment, as courts have broadly upheld taxes in the face of Takings Clause challenges," reasoning that the general benefits of a functioning society are treated as the "compensation" taxpayers receive.

That does not make the Takings Clause wholly irrelevant. Legal commentary tracing the U.S. Supreme Court's 1960 decision in Armstrong v. United States notes that a tax can cross into unconstitutional-taking territory if it forces a narrow class of people "alone to bear public burdens which, in all fairness and justice, should be borne by the public as a whole"—precisely the argument available against a levy aimed at roughly 200 named taxpayers. Baker Botts attorneys, in a client analysis of the initiative, concluded that taxpayers "could argue" the act violates the Takings Clauses of both the federal and California constitutions on those grounds, while acknowledging that California's Takings Clause is interpreted consistently with the federal one and that courts have historically been reluctant to strike down revenue measures on this basis. A separate analysis by LegalClarity concurs that the takings argument "gains traction at extreme rates" compounding over time, but calls it "a secondary argument, unlikely to succeed against modestly-rated proposals," functioning more as an outer ceiling on how aggressive a wealth tax can be than as a likely basis for invalidating a one-time 5 percent levy.

The Heritage Foundation has made a more categorical Takings Clause argument against wealth taxation generally, arguing in the context of Sen. Elizabeth Warren's proposed federal wealth tax that "seizing this private property without just compensation would clearly violate the Fifth Amendment Takings Clause." That federal proposal, however, differs structurally from Proposition 40: it would be a recurring annual tax on wealth above $50 million nationally, not a one-time state excise tax, and it also faces separate apportionment problems under Article I that do not apply to state taxation.

Stronger Legal Vulnerabilities, According to Tax Attorneys

Multiple law firms and research organizations that have reviewed the initiative's text rank other constitutional theories as more likely to succeed than a Takings Clause claim:

Retroactivity and Due Process. The tax attaches to anyone who was a California resident on Jan. 1, 2026—roughly ten months before voters even cast ballots on the measure. Wealth-management firm Capital Group's legal summary calls this "the strongest challenge," noting that "courts have historically been skeptical of taxes that create liabilities before the taxpayer could have known the law existed, particularly for a type of tax that has never existed before." Baker Botts's analysis notes the U.S. Supreme Court has suggested the ordinary rational-basis tolerance for modest retroactivity "may not apply" to "the creation of a wholly new tax." Tax attorney Jon Feldhammer told CNBC that departing billionaires who left after Jan. 1 but before the November vote may have the strongest individual claims.

California's 0.4 percent property-tax cap. Article XIII, Section 2, of the state constitution limits taxes on stocks, bonds, and similar intangible property to four-tenths of one percent of value. Baker Botts notes that although the initiative is labeled an "excise tax" to sidestep this cap, "a court could classify it as a property tax based on the reasoning that labels are not conclusive for this determination."

Dormant Commerce Clause. Under the four-part Complete Auto Transit v. Brady (1977) test, a state tax must be fairly apportioned and nondiscriminatory toward interstate commerce. Because the measure taxes worldwide net worth rather than only wealth accumulated in California, the Tax Foundation's analysis argues the "default rule that the tax is not prorated by residency history is in obvious tension with the fair apportionment prong," particularly for a billionaire who built a fortune elsewhere and only recently moved to California.

Bill of Attainder. Article I, Section 10, of the U.S. Constitution bars states from singling out a specific, named class of people for punishment without trial. Baker Botts observes the initiative "targets a group of 'around 200 billionaires,'" and while taxes are not usually treated as "punishment," opponents could argue the framing of the measure evidences punitive intent.

Right to Interstate Travel. Because the tax can reach a person who resided in California only briefly, Baker Botts also flags a potential challenge under Shapiro v. Thompson, which held that laws penalizing the exercise of interstate travel are unconstitutional absent a compelling state interest.

The initiative's own drafters anticipated litigation: it includes an unusually broad severability clause instructing courts to modify the Jan. 1 and Dec. 31 dates if either is struck down, and it authorizes an expedited direct appeal to the California Supreme Court within 60 days of the vote—bypassing the traditional "pay first, sue later" doctrine that normally governs state tax disputes.

The Money and the Politics

Forbes-derived estimates place the collective wealth of California's roughly 213 billionaires at $2.18 trillion, and the measure's drafters project $100 billion in revenue over five years after accounting for an assumed 10 percent avoidance rate. A March 2026 study from Stanford's Hoover Institution, led by senior fellow Joshua Rauh, estimated significantly lower net revenue—around $40 billion in direct wealth-tax collections—and projected the state would ultimately be roughly $25 billion worse off once lost income-tax revenue from departing high earners is factored in.

Opposition, organized under the campaign "Building a Better California," raised more than $80 million in the first quarter of 2026 alone, funded substantially by California billionaires, including reported backing from Google co-founder Sergey Brin. Opponents also filed five competing constitutional-amendment initiatives designed to blunt the measure, including proposals to raise the voter-approval threshold for one-time taxes to two-thirds and to bar new taxes on retirement savings.

Gov. Newsom has been the measure's most prominent Democratic opponent, telling Politico the plan "makes no sense" and is "really damaging to the state." Several Democratic gubernatorial candidates, including Katie Porter and Xavier Becerra, also oppose it, while Sen. Bernie Sanders (I-Vt.) and Rep. Ro Khanna (D-Calif.) have campaigned in favor. Nvidia CEO Jensen Huang said an estimated $8 billion personal liability was "perfectly fine" with him, while investor Peter Thiel publicly cited his established Miami residency amid reports that at least two other unnamed California billionaires have relocated or begun relocation plans since the Jan. 1 residency date passed.

Public polling has shown majority support for the concept alongside broad skepticism about its durability: an early-2026 Berkeley IGS/Politico poll and subsequent surveys found a majority of voters expect billionaires to find ways around the tax, and roughly half expect the measure to be tied up in litigation for years regardless of the outcome at the ballot box.

What Happens Next

If voters approve Proposition 40 in November, the earliest constitutional challenges are expected to be filed within days, given the initiative's own 60-day litigation window and its unusual routing directly to the California Supreme Court. Attorneys interviewed by CNBC and law firms including Baker Botts and Pillsbury say a definitive ruling could plausibly arrive before the April 15, 2027, payment deadline—meaning the fate of the nation's first major state-level wealth tax may be decided by justices well before most billionaires would ever have to write a check.

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Sources

1. Galle, Brian; Gamage, David; Saez, Emmanuel; Shanske, Darien. Expert Report on the California 2026 Billionaire Tax: Revenue, Economic, and Constitutional Analysis. December 2025. https://eml.berkeley.edu/~saez/galle-gamage-saez-shanskeCAbillionairetaxDec25.pdf
2. Institute on Taxation and Economic Policy (ITEP). "Expert Report on the California 2026 Billionaire Tax: Revenue, Economic, and Constitutional Analysis." https://itep.org/expert-report-on-the-california-2026-billionaire-tax-revenue-economic-and-constitutional-analysis/
3. Office of the California Attorney General. Initiative 25-0024, Amendment No. 1 ("2026 Billionaire Tax Act"), official initiative text. https://oag.ca.gov/system/files/initiatives/pdfs/25-0024A1%20(Billionaire%20Tax%20).pdf
4. Wikipedia contributors. "2026 California billionaire tax initiative" / "2026 California Proposition 40." Last updated July 2026. https://en.wikipedia.org/wiki/2026_California_billionaire_tax_initiative and https://en.wikipedia.org/wiki/2026_California_Proposition_40
5. Ballotpedia. "California One-Time Wealth Tax for State-Funded Healthcare, Education, and Food Assistance Programs Initiative (2026)." https://ballotpedia.org/California_One-Time_Wealth_Tax_for_State-Funded_Healthcare,_Education,_and_Food_Assistance_Programs_Initiative_(2026)
6. Ballotpedia News. "Signatures submitted for California initiatives to prohibit taxes on personal savings and assets, require audits for new special taxes, and challenge proposed billionaire tax measure." May 7, 2026. https://news.ballotpedia.org/2026/05/07/signatures-submitted-for-california-initiatives-to-prohibit-taxes-on-personal-savings-and-assets-require-audits-for-new-special-taxes-and-challenge-proposed-billionaire-tax-measure/
7. Tax Foundation. "2026 California Billionaire Tax Act: Details & Analysis" (legal-challenges research page). Updated May 26, 2026. https://taxfoundation.org/research/all/state/california-billionaire-wealth-tax-legal-challenges/
8. Tax Foundation. "California Wealth Tax | 2026 Billionaire Tax Act." April 30, 2026. https://taxfoundation.org/blog/california-wealth-tax/
9. Baker Botts L.L.P. "California 2026 Billionaire Tax Act." Thought Leadership, December 2025. https://www.bakerbotts.com/thought-leadership/publications/2025/december/california-2026-billionaire-tax-act
10. Baker Botts L.L.P. "Update on the California 2026 Billionaire Tax Act." Thought Leadership, May 2026. https://www.bakerbotts.com/thought-leadership/publications/2026/may/update-on-the-california-2026-billionaire-tax-act
11. Pillsbury Winthrop Shaw Pittman LLP (SeeSALT blog). "California's Amended Wealth Tax Initiative Meets Five New Foes." December 15–16, 2025. https://seesalt.pillsburylaw.com/californias-amended-wealth-tax-initiative-meets-five-new-foes/
12. National Taxpayers Union Foundation. "California Wealth Tax Proposal Achieves a New Feat in Tax Policy: Losing the State Money Before It Even Becomes Law." February 19, 2026. https://www.ntu.org/foundation/detail/california-wealth-tax-proposal-achieves-a-new-feat-in-tax-policy-losing-the-state-money-before-it-even-becomes-law
13. LegalClarity. "Is a Wealth Tax Unconstitutional? What Courts Say." April 1, 2026. https://legalclarity.org/is-a-wealth-tax-unconstitutional-under-the-u-s-constitution/
14. The Heritage Foundation. "Elizabeth Warren's 'Wealth Tax' Has Three Strikes Against It — Here's Why." April 30, 2019. https://www.heritage.org/taxes/commentary/elizabeth-warrens-wealth-tax-has-three-strikes-against-it-heres-why
15. Hoover Institution. "California's Proposed Billionaire Tax Will Cost the State an Estimated $25 Billion, Hoover Study Finds." March 5, 2026 (Rauh, Jaros, Kearney, Doran, Cosso). https://www.hoover.org/press/californias-proposed-billionaire-tax-will-cost-state-estimated-25-billion-hoover-study-finds
16. Capital Group. "What is the California 2026 Billionaire Tax Act?" https://www.capitalgroup.com/pcs/insights/articles/ca-billionaire-tax-2026.html
17. CNBC (Inside Wealth newsletter). "California's controversial wealth tax proposal leaves billionaires with little way out." January 8, 2026. https://www.cnbc.com/2026/01/08/california-wealth-tax-proposal-leaves-billionaires-with-little-way-out.html
18. CNBC. "Nvidia's Jensen Huang would have to pay about $8 billion in proposed billionaire tax — he says that's 'perfectly fine' with him." January 7, 2026 (Tom Huddleston Jr.).
19. Fortune. "'Don't leave': Jensen Huang challenges billionaire class as he insists 'highest taxes in the world' are OK with him." April 23, 2026 (Jacqueline Munis).
20. Politico. "Newsom unloads on California wealth tax proposal: 'Makes no sense.'" January 12, 2026.

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Note: This article addresses a contested, ongoing political and legal dispute. Constitutional predictions cited above reflect the professional judgment of the named law firms, think tanks, and researchers as of mid-2026; no court has yet ruled on the merits of Proposition 40, and its legal fate will not be resolved until after the November 2026 election, if the measure passes.