The Legal Challenges to California’s Proposition 40, the 2026 Billionaire Tax Act

The 2026 Billionaire Tax Act, (now officially known as Proposition 40) on the November 3, 2026 ballot, aims to impose a 5% tax on the net worth of California residents (and certain trusts) worth $1 billion or more. It has already qualified for the ballot.

Who would owe this tax?

Liability for this tax hinges on state residency as of January 1, 2026, with valuation as of December 31, 2026.

Proceeds would primarily fund healthcare, with smaller shares for education and food assistance, funneled through a specially crafted reserve fund meant to sidestep various constitutional limits.

The proposal is not without ambition. Yet its legal vulnerabilities remain substantial under both the U.S. and California Constitutions, inviting serious judicial scrutiny that could reshape – or derail – its implementation.

Retroactivity Concerns

At its core, the measure locks in tax obligation based on a residency snapshot taken months before the election. While valuation occurs at year-end 2026 and the initiative cleverly frames the levy as an “excise tax… on the activity of sustaining excessive accumulations of wealth,” the pre-vote residency trigger still creates new liabilities with limited advance notice for affected individuals.

Due process precedents allow some retroactive taxation when supported by a rational purpose and modest reach. But applying an entirely new wealth-based excise tax with this timing raises legitimate questions about fairness and surprise. The initiative’s severability and date-reformation clauses offer a safety valve, but they do not erase the underlying due process tension.

Dormant Commerce Clause Issues

The tax reaches worldwide net worth, encompassing assets and value created far beyond California’s borders. The measure tries to thread the needle with 100% residency-based apportionment and a petition process for alternative methods (subject to a general 25% floor).

Even so, this approach risks running afoul of the Dormant Commerce Clause and the Complete Auto test by imposing burdens on interstate commerce without traditional fair apportionment for intangibles and out-of-state wealth.

Federal courts have invalidated less sweeping extraterritorial efforts. This remains one of the proposal’s more exposed provisions and highly likely to be challenged.

Bill of Attainder and Equal Protection Considerations

By zeroing in on roughly 200 individuals and trusts above the $1 billion threshold (with a phase-out between $1 billion and $1.1 billion), the measure invites claims that it singles out a small, identifiable class for special treatment. “Bill of Attainder” doctrine is narrow and requires clear punitive intent.

Still, the narrow targeting and substantial one-time hit provide rhetorical and legal ammunition for arguments that these veers closer to punishment than neutral taxation.

Takings Clause Arguments

Requiring payment of 5% of net worth – including illiquid startup equity or family business interests – raises takings questions, especially when combined with retroactive elements. The initiative offers installment payments (with a 7.5% deferral charge) and limited deferral relief for illiquid assets, which helps.

Taxes are rarely deemed takings, but the scale, selectivity, and burden on a tiny group could test where a legitimate tax ends and an uncompensated taking begins.

California Constitutional Issues: Amendment vs. Revision and Single-Subject Rule

The initiative adds Section 37 to Article XIII of the California Constitution to authorize the tax, override certain property and appropriations limits, and establish the special fund. This explicit amendment bolsters its defenses, yet it could still be challenged as a de facto revision that fundamentally alters the state’s taxation and budgetary framework.

Separately, bundling the tax imposition, detailed spending directives, constitutional overrides, and expedited review procedures into one measure tests the single-subject rule. While all provisions arguably relate to funding specific programs through billionaire net worth taxation, the package’s scope leaves room for “logrolling” arguments.

Likely Post-Election Litigation and Practical Challenges

Even if Proposition 40 passes, its journey would likely be far from over. Valuation battles over private companies, intellectual property, trusts, and complex holdings would proliferate and drag on. Questions involving former California residents, outbound moves, and the proper reach of the tax would multiply. The measure (helpfully) builds in an expedited validation process in Sacramento Superior Court with direct appeal to the California Supreme Court, an admission that courtroom drama circus is expected.

Administrative realities would compound the issues. California has no mature system for routinely appraising ultra-high-net-worth illiquid assets at fair market value. The resulting disputes could tie up billions in disputed revenue for years, creating precisely the fiscal uncertainty the measure seeks to resolve.

Who can challenge Prop 40 and how?

Any interested party (e.g., affected taxpayers, advocacy groups, or even the state itself) can file a writ petition in California Superior Court (often Sacramento County for statewide measures) or directly seek review in the California Supreme Court.

Proposition 40 reflects a bold attempt to address budget pressures by tapping a narrow slice of extreme wealth. Its drafters included several safeguards to improve survivability, yet the retroactive residency hook, worldwide reach, narrow targeting, and structural overrides still present formidable constitutional obstacles.

No pre-election challenges to Proposition 40 so far – most likely is strategic patience rather than any lack of opposition.

Major opponents appear to be prioritizing voter rejection, campaign spending, and competing measures over immediate litigation. The window for a pre-election writ petition remains open, with the strongest opportunity in the coming weeks before final ballot certification, and a surprise filing from a well-resourced plaintiff is still possible.

This pattern is familiar in high-stakes California ballot fights: the real legal battle often begins only after the election. If new developments emerge, such as a major plaintiff announcement, the situation could shift rapidly.

Major opponents, including billionaires and business interests, may be avoiding court fights at this stage for several reasons: A lawsuit now could unify progressive voters behind the measure by framing it as an attempt by the wealthy to silence the people; It risks painting the challengers as anti-democratic; In addition, some moderate Democrats appear to prefer contesting the proposal at the ballot box or through competing measures already in play.

The measure only recently qualified on June 17, 2026. Drafting a strong writ petition, securing plaintiffs with standing such as affected billionaires or trusts, assembling expert declarations, and selecting the proper venue all require time. Legal teams may still be refining strategy, awaiting further formal steps from the Secretary of State, or monitoring any lingering signature validity issues.

Pre-election invalidation is rare, but robust post-election challenges are all but guaranteed. In the end, the very features that make the tax politically appealing may ensure it remains mired in litigation long after any vote. It’s a high-stakes gamble for a state that can ill afford prolonged uncertainty.