California’s proposed 2026 Billionaire Tax Act would impose a one-time wealth tax on billionaires tied to California residency, but the measure’s residency and valuation rules may create major legal vulnerabilities, especially for billionaires who leave the state during 2026.
How the Proposed California Wealth Tax Would Work
The 2026 California Billionaire Tax Act would impose a one-time 5% tax on the global net worth of billionaires who were California residents as of January 1, 2026.
The measure would value taxable wealth as of December 31, 2026.
Because the initiative would go before voters in November 2026, the residency date would come more than ten months before adoption of the tax.
The source argues that the retroactive structure is not automatically unconstitutional, but that the specific residency and valuation rules make the initiative vulnerable to legal challenge.
The measure’s drafters appear to have anticipated possible legal problems. The initiative includes mechanisms for alternative apportionment if assigning 100% of the tax base to California is ruled unlawful or unfair. It also asks courts to reform the January 1 and December 31 dates if they are found unconstitutional, rather than striking down the entire measure.
Why Mid-Year Movers Matter
The initiative treats California residency as a one-time snapshot event on January 1, 2026.
Under ordinary California income tax residency rules, a person who starts the year as a California resident but establishes residency elsewhere later in the year would generally be treated as a part-year resident. They would owe California income tax only for the portion of the year spent in the state.
The proposed wealth tax works differently. A taxpayer who leaves California during 2026 could still be taxed on their entire global net worth, with no automatic apportionment.
The tax could also apply to wealth that appreciated or was acquired after the taxpayer left California, because the valuation date is December 31, 2026.
This means a person who was a California resident on January 1, moved away later in 2026, and only crossed the $1 billion net worth threshold after leaving could still be subject to the tax, even though they were not a billionaire while residing in California.
The Residency Problem
The initiative adopts California income tax residency standards but changes how they operate by using a single January 1 snapshot date.
California residency normally depends on a “closest connection” analysis, including:
- Time spent in California
- Family ties
- Property ties
- Professional connections
- Social connections
- Other personal and business links
The initiative’s proponents argue that simply buying property elsewhere, opening out-of-state office space, or announcing a planned move would not necessarily sever California tax residency. California residents are presumed to remain residents until they can show that their absence is not temporary.
If the measure passes and the January 1 residency date is upheld, residency disputes could become fact-intensive. Courts may need to determine whether people who left in late 2025 or during 2026 had actually established domicile elsewhere.
The source argues that the initiative may function not only as retroactive to January 1, 2026, but effectively to some point before then, because taxpayers may need to have severed California ties before the January 1 snapshot date to avoid exposure.
Retroactivity and a “Wholly New Tax”
The source identifies retroactivity as one of the main legal issues.
Retroactive taxation is not automatically unconstitutional. Courts have upheld some retroactive tax changes, especially where the retroactivity was limited and tied to correcting or modifying existing tax rules.
However, the source argues that the California proposal may be more vulnerable because it would retroactively impose a wholly new tax.
The article discusses older U.S. Supreme Court gift tax cases, Blodgett v. Holden and Untermyer v. Anderson, where the Court struck down retroactive applications of a new tax to transactions that occurred before enactment.
In Untermyer, the Court stated that taxpayers should know when and how they become liable for taxes and should not be required to guess the outcome of pending legislation.
The source argues that this is relevant because the California wealth tax would not merely modify an existing tax. It would create a new tax and apply it based on a residency date before voter approval.
The article notes that later cases, including United States v. Carlton, allowed retroactive tax changes but did not clearly overturn the older distinction between retroactive changes to existing taxes and retroactive imposition of a wholly new tax.
If that distinction remains valid, the source argues, the California wealth tax’s retroactivity may fail.
Right-to-Travel Concerns
The initiative’s January 1 snapshot date may also raise right-to-travel issues.
The source argues that the date appears designed to limit the ability of billionaires to move away before becoming subject to the tax. If a court views the provision as penalizing or restricting interstate movement, the measure could face stricter constitutional scrutiny.
The source cites the principle that movement between states is a fundamental right. If the tax structure is understood as attempting to prevent taxpayers from avoiding the tax by moving, the state may need to show a compelling government interest.
The Post-Departure Wealth Problem
The initiative also creates legal questions because it separates the residency date from the valuation date.
A taxpayer could be considered liable based on January 1 residency, but the tax would be calculated based on wealth as of December 31.
The source argues that this raises two problems:
- The tax is not automatically apportioned for people who leave California during the year.
- The tax could apply to wealth acquired or accumulated after departure.
States can generally tax residents on worldwide income, but the source notes that due process requires a minimum connection between the state and the person, property, or transaction being taxed.
Once a person establishes domicile outside California, the source argues that California’s authority to tax wealth assessed later in the year becomes doubtful, especially for wealth with no connection to California.
Commerce Clause and Apportionment Issues
The source also discusses possible dormant Commerce Clause challenges.
Under the Complete Auto Transit v. Brady framework, state taxes must satisfy requirements including:
- Substantial nexus
- Fair apportionment
- Nondiscrimination
- Fair relationship to services, benefits, and protections provided by the state
The source argues that taxing 100% of a taxpayer’s wealth as of December 31, without adjustment for part-year residency or post-departure wealth, conflicts with fair apportionment principles.
The initiative allows taxpayers to seek a “constitutional alternative” apportionment, but the source argues that this may not be enough. Requiring each taxpayer to challenge the default rule could shift the litigation burden onto taxpayers while treating full California apportionment as the starting point.
The source also compares the structure to an exit tax, because it may deny would-be movers the practical benefit of leaving California during 2026.
Judicial Reformation May Be Difficult
The initiative asks courts to modify its residency and valuation dates if the default dates fail constitutional review.
However, the source notes that California courts have often been reluctant to rewrite tax statutes through judicial reformation.
Under Kopp v. Fair Political Practices Commission, courts may reform a statute if they can confidently conclude that:
- The statute can be reformed in a way that closely reflects the enacting body’s policy choices.
- The enacting body would have preferred the reformed version to invalidation.
But in cases involving tax laws, including Abbott Laboratories v. Franchise Tax Board, Ventas Finance I v. Franchise Tax Board, and Ceridian Corp. v. Franchise Tax Board, California courts have refused to use judicial reformation to cure constitutional defects.
The source argues that rewriting the wealth tax could require courts to choose between multiple policy options, such as:
- Moving the residency date to voter approval day
- Moving the residency date to certification day
- Aligning the residency and valuation dates
- Moving the tax to January 1, 2027
- Replacing snapshot residency with part-year apportionment
- Excluding post-departure wealth
- Reworking the tax as a whole
Because these choices could have very different fiscal and policy effects, courts may decide that the matter belongs to lawmakers or voters rather than judges.
The more constitutional defects courts identify, the less plausible judicial reformation may become, increasing the chance that the tax could be struck down rather than rewritten.
What This Means for Billionaires Leaving in 2026
The text of the initiative says that billionaires who leave California in 2026 would have left too late to avoid the tax. It may also attempt to reach some people who left in late 2025.
The source argues that those provisions are legally uncertain.
Potential challenges could include:
- A facial challenge that the tax is an unconstitutional retroactive imposition of a wholly new tax
- An as-applied challenge by someone who was no longer a California resident on the December 31 valuation date
- A challenge to the lack of apportionment for part-year residents
- A challenge to taxation of wealth acquired or accumulated after leaving California
- A claim that the structure interferes with the right to travel
- A Commerce Clause challenge based on lack of fair apportionment or insufficient nexus
If the tax passes but courts reform the dates, some people who leave in 2026 may avoid some or all liability depending on how the measure is rewritten or applied.
Practical Effect Before the Vote
The source argues that the initiative may encourage billionaire departures even before voters decide on it.
Because the measure appears electorally viable and contains provisions intended to lock taxpayers into the base, wealthy Californians may have an incentive to establish domicile in another state before the election or before later court rulings.
If voters approve the measure, the source expects serious litigation over the tax’s constitutionality and over which taxpayers can be taxed.
The tax could be struck down entirely, or it could survive in a narrower form that limits or eliminates liability for people who leave after January 1, 2026.
The main issue is that the initiative tries to tax billionaires based on a January 1 residency snapshot while valuing wealth on December 31. That structure is designed to prevent taxpayers from escaping the tax by moving, but the source argues that these same provisions create the strongest legal vulnerabilities.
Source article: taxfoundation.org
