The proposal, backed by the Service Employees International Union Healthcare Workers West, would impose a one-time, 5% tax on individuals whose net worth exceeds $1 billion and who were living in the state as of Jan. 1, 2026. The goal is to generate $100 billion in revenue, which would largely be used to offset federal funding cuts to healthcare for low-income people.

“California’s health is at stake,” said Liz Perlman, executive director of a chapter of the American Federation of State, County and Municipal Employees, a major labor union. “Hospitals are closing and people will die. Why? So billionaires can get another tax cut that they don’t need.”

The California Secretary of State still has to verify the signatures and officially place the measure on the ballot. Backers say they collected more than 1.5 million signatures, well over the roughly 875,000 they needed. California allows ballot initiative campaigns to pay people per signature they gather. The cost of gathering petition signatures can vary widely, but it typically runs around $15 for each signature.

If the measure goes before voters in November, it could prompt one of the costliest ballot fights ever and will draw national attention as a litmus test for voter attitudes on raising taxes on the rich. Vermont Sen. Bernie Sanders has campaigned in support of the idea. Meanwhile, Google founder Sergey Brin has already donated $57 million to a political committee called “Building a Better California” that’s backing a variety of initiatives designed to blunt the billionaires’ tax. It’s raised over $90 million, counting Brin’s contributions, from fewer than a dozen donors.

Democratic Gov. Gavin Newsom and Silicon Valley tech moguls are adamantly opposed. They warn it will drive California’s wealthiest residents out of the state. Nearly half of California’s personal income tax revenue comes from the top 1% of earners. Some have already purchased properties out of state in case it passes.

“After playing with matches since October, the SEIU has succeeded in lighting a ‘Tax the Rich’ wildfire by getting enough signatures,” said David Lesperance, a tax consultant who’s advised some of his wealthy clients who left California because of the proposal. “The many billionaire targets of their efforts have already responded by executing fire escape plans by relocating to other states.”

Brian Brokaw, a longtime Newsom adviser who is leading a political committee opposing the tax, said the measure was poorly constructed and would deal a huge blow to the state’s budget.

“Enacting a so-called wealth tax in just one state wouldn’t target a small group — it would impact all 40 million Californians,” he said in a statement. “This proposal trades a short-term revenue bump for long-term losses.”

At least 25 billionaires listed among Forbes magazine’s 2025 rankings of the world’s 500 wealthiest people either lived in California or had some significant ties to the state, based on a review by The Associated Press. But determining whether they were full-time residents or just frequent visitors could turn into a matter of dispute, since many of them own property elsewhere.

The big tax and spending cuts law President Donald Trump signed last year will cut more than $1 trillion nationwide over a decade from Medicaid and federal food assistance.

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Associated Press writer Michael R. Blood in Los Angeles contributed.