Opponents of Proposition 40, California's one-time 5 per cent tax on billionaires, have spent more than $304 million against it before the 3 November vote, a Capital & Main analysis of state records through 20 October found. The analysis was published on 22 October; the total is that outlet's tally, and this publication has not rebuilt it from the filings.
The fight over the tax accounts for nearly half of all money spent on California's ballot measures this year, by the same analysis. Measured against what the tax is meant to raise, it is small. That gap explains both why the money is being spent and why the outcome is hard to call.
What would Proposition 40 do?
It would charge each California billionaire a single tax equal to 5 per cent of net worth. According to the Legislative Analyst's Office analysis in the official voter guide, the tax applies to people who were California residents on 1 January 2026, is due in 2027, and can be spread over five years, at extra cost to the taxpayer. Real estate, pensions and retirement accounts are generally excluded. The analyst's office puts the number of people affected at "a few hundred"; CalMatters put it at roughly 200.
Ninety per cent of the money must go to public health care services, with the rest for education, food assistance and running the tax. The analyst notes that the revenue would sit outside the state's constitutional spending limit and its school-funding guarantee, which the voter guide summary states in its first line.
The sponsor is the Service Employees International Union-United Healthcare Workers West. Its spokesperson, Renée Saldaña, told CalMatters: "Nobody else has a solution to fill this massive $100 billion funding gap that is facing California." Governor Gavin Newsom opposes the measure, CalMatters reported.
How much would it raise?
Nobody knows, and the official estimate says so. The analyst's office says the state "probably would collect tens of billions of dollars," spread over several years, and that the exact amount and timing are "very hard to predict." Supporters use $100 billion. The analyst also expects departures and other responses to cut ongoing state income tax revenue by "less than $1 billion per year," because billionaires who leave stop paying California income tax too.
That last point is the quiet trade-off in the measure. A one-time gain is set against a possible permanent, smaller loss. The voter guide does not say how the two net out over a decade, and we have not tried to; the inputs are not public.
Who is paying to stop it?
Mostly one committee. Capital & Main found that Building a Better California, a political action committee, has spent more than $304 million: $118.5 million against Proposition 40 directly and $186 million in support of Propositions 41 and 42, two rival measures. About 96 per cent of opposition spending came through it. Sergey Brin, the Google co-founder, is its largest donor at $102 million, followed by Eric Schmidt at $40.2 million and L. John Doerr at $40 million, according to the analysis. The union has contributed nearly $32 million in support.
| Measure of the campaign | Figure |
|---|---|
| Spent by Building a Better California | more than $304 million |
| Of which against Prop 40 directly | $118.5 million |
| Of which for Props 41 and 42 | $186 million |
| Contributed by SEIU-UHW in support | nearly $32 million |
| Opposition spending per affected taxpayer (about 200) | about $1.5 million |
| Opposition spending as share of supporters' $100 billion estimate | about 0.3 per cent |
Source: Capital & Main analysis of state records through 20 October 2026; CalMatters. The last two rows are this publication's arithmetic.
By this publication's arithmetic, the opposition has outspent the sponsor by roughly 9.5 to 1, and its spending so far equals about 0.3 per cent of the revenue the sponsor says the tax would raise. Against the analyst's lower "tens of billions," the share would be higher, perhaps around 1 per cent at $30 billion, but still small. For the people who would owe the tax, the campaign is cheap insurance. That is our reading, not either side's account.
Why could it fail even if it wins?
Because of the two measures the same money is promoting. Proposition 41 would block new state taxes whose revenue sits outside the spending limit, as Proposition 40's does, and Proposition 42 would ban new state taxes on personal property and assets, CalMatters reported. Under the state constitution, when competing measures both pass, the one with more yes votes prevails. The analyst's office warns that Proposition 40 "could be stopped from becoming law even if it gets yes votes from a majority of voters."
The polling leaves all three outcomes open.
| Poll | Fieldwork | Yes | No | Undecided |
|---|---|---|---|---|
| PPIC, likely voters | released 15 September | 52% | 46% | n/a |
| UC Berkeley IGS, likely voters | mid-September | 45% | 42% | 13% |
| NBC News, likely voters | 21-24 September | 48% | 38% | 14% |
Sources: CalMatters (PPIC); Capital & Main (IGS, NBC News).
The same PPIC survey found 51 per cent support for Proposition 41 and 54 per cent for Proposition 42, according to CalMatters. If those figures held, both rivals would pass with more yes votes than the tax.
Legal risk follows even a clean win. CalMatters reported that the measure's reach back to 1 January 2026 invites a constitutional challenge, which supporters reject, and Robert Lapsley of the California Business Roundtable objects to the Legislature's power to amend it later: "They can change the level of taxation; they can change how often they get taxed." The union disputes that reading.
What to watch on 3 November
Two numbers, not one. Proposition 40 needs a majority, and it needs more yes votes than both Proposition 41 and Proposition 42. Health care is where the money would go, at a time when Washington's interest bill rivals what it sends the states for Medicaid, and Sacramento has a public-sector labour dispute of its own; how it would spend a windfall is a question for later. Whether there is one will be settled by the ranking of three yes counts, and then, very likely, by a court.
The measure's terms, the fiscal estimate, the exclusions, the departure effect and the conflict with Propositions 41 and 42 are from the Legislative Analyst's Office analysis in the official California voter guide for the 3 November 2026 election. Campaign spending, donor totals and the IGS and NBC News polls are from Capital & Main's analysis by Ben Poston of state campaign finance records through 20 October 2026, published on 22 October 2026. The $100 billion supporters' estimate, the roughly 200 taxpayers, the 90/10 spending split, Governor Gavin Newsom's opposition and the quotes from Renée Saldaña and Robert Lapsley are from CalMatters' report of June 2026. The PPIC September survey results are from CalMatters' report of 16 September 2026. Spending as a share of the estimated revenue, the spending ratio and spending per affected taxpayer are this publication's arithmetic. The analysis is our own.
