Backers of a new tax on California billionaires appear to have gathered more than enough signatures to put the proposal on the ballot this fall, escalating a battle between the state’s labor unions and its richest residents.

The labor coalition behind the initiative said at a press conference Friday that more than 1.5 million Californians had signed the petition calling for a one-time 5% tax on the assets of people with at least $1.1 billion. They would need at least 875,000 valid signatures to put the question to voters in November.

Backers planned to submit the signatures to the state on Friday so officials could begin the verification process.

Mayra Castañeda, a member of SEIU-United Healthcare Workers West, the union leading the campaign, said approval of the tax would help offset healthcare funding cuts created by last year’s Republican tax bill.

“California is home to more billionaires than anywhere in the country,” Castañeda said. “Ultra-wealthy billionaires have seen their fortunes skyrocket, even as food, rent and gas prices increase, crushing working families who are struggling to stay afloat.”

The signature filing, she added, would bring them “one step closer to the ballot.”

The proposal has prompted fierce pushback from some of the state’s wealthiest tech executives, including Google co-founder Sergey Brin, who’s estimated to be worth $273 billion. Brin moved several of his business entities to Nevada late last year in an apparent effort to limit his exposure to the tax. If approved by voters, the tax would apply to billionaires who resided in California as of Jan. 1, 2026.

Though the measure has the support of some high-profile politicians on the left, including Vermont Sen. Bernie Sanders (I), it does not have the backing of California Gov. Gavin Newsom. The Democrat and possible 2028 White House hopeful has opposed the measure on the grounds it would push wealthy residents out of the state and thereby weaken California’s tax base.

“This will be defeated — there’s no question in my mind,” Newsom recently told The New York Times. “I’ll do what I have to do to protect the state.”

In a poll last month, half of voters said they would back the measure, while 28% would oppose it. Yet 54% said they were concerned about wealthy residents like Brin packing up and leaving the state. Rich opponents of the measure are expected to spend enormous amounts of money on counter-messaging in the coming months.

The campaign backing the tax projects it would raise about $100 billion over five years, with 90% of the money devoted to healthcare and the remainder to education and food assistance. The residents subject to the tax would be allowed to pay it all in one lump sum or in annual installments over five years.

Forbes’ billionaire list includes more than 200 people in California with a combined wealth of more than $2 trillion, according to the Institute on Taxation and Economic Policy, a left-leaning think tank. The group estimated an “avoidance rate” on the tax of just 10%, since the number of billionaires is relatively small and therefore “each taxpayer can be carefully audited.”

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