The most sympathetic faces in the campaign against California's billionaire tax are nurses, teachers, firefighters and building-trades workers. State campaign-finance filings show the single largest check keeping their coalition running came from the billionaires' own committee.

The "No on 40" coalition markets itself as a working-class revolt against Proposition 40, the wealth tax aimed at California's roughly 200 billionaires. But state disclosure records show its biggest donor is Building a Better California — the Sergey Brin-funded operation that has stockpiled more than $150 million to defeat the measure. The money lands at a bad moment for the opposition: the California Democratic Party and the state's largest labor federation have both swung behind the tax and against the billionaires' countermeasures, leaving the grassroots face of the campaign financially roped to the money it exists to soften.

Proposition 40 would charge a one-time 5% levy on Californians who crossed the billion-dollar mark this past January — an estimated 200 people — and steer the proceeds, projected at roughly $100 billion over five years, mostly toward healthcare, with slices for food aid and education, per CalMatters' voter guide (external source, opens in a new tab). Its sponsor is the healthcare union SEIU-United Healthcare Workers West. One wrinkle is baked into the text: if either of two rival measures — Propositions 41 and 42 — outpolls it, Prop 40 dies.

Those countermeasures are the billionaires' insurance policy, and the organizations pushing them overlap with the coalition branded "No on 40." The voter guide lists the tax's opponents as the California Medical Association, the California Teachers Association, Planned Parenthood, the California Professional Firefighters and the State Building and Construction Trades, alongside crypto financier Chris Larsen and venture capitalist Ron Conway. The roster is genuinely broad. The records suggest it is also underwritten.

Building a Better California has reported more than $150 million, the overwhelming share of it from Google co-founder Sergey Brin, in state top-contributor filings (external source, opens in a new tab). Those same disclosures show the committee spreading money across the ballot — and, tellingly, sitting atop the donor list of the very No on 40 committee that fronts the working-class opposition, with a $5 million contribution. In other words, the largest single check the "teachers and hard hats" coalition has banked was written by the billionaires it ostensibly stands apart from.

That arrangement looked airtight until the endorsements turned. On Aug. 2, the California Democratic Party's executive board voted 61.7% — a hair above its 60% bar — to back Prop 40 and oppose Props 41 and 42, KQED reported (external source, opens in a new tab), breaking with Gov. Gavin Newsom. Two days later the California Federation of Labor Unions, which speaks for 1,300 unions and 2.3 million members, threw in behind the tax (external source, opens in a new tab).

Federation president Lorena Gonzalez tied the measure to federal Medicaid cuts, argued the state's billionaires should refund California's hospitals, and dismissed the opposition's complaints without ceremony.

The twin endorsements strip the opposition of the progressive cover it had been building. What remains, in the filings, is the arithmetic: a nine-figure war chest bankrolled almost entirely by one Silicon Valley fortune, routed in part through a coalition designed to keep that fortune out of the frame. For a campaign built to put teachers and hard hats — not billionaires — out front, the disclosure trail is an awkward thing to explain to voters this fall.