U.S. Senator Bernie Sanders is now backing the campaign.
Meanwhile, some big-name billionaires have reportedly threatened to leave the state.
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State labor groups are calling for a one-time 5 percent emergency tax on several of California's wealthiest residents.
"We are calling on California's approximately 200 billionaires to step up," said Renee Saldana, a spokesperson for SEIU United Health Care Workers West labor union.
Many of those billionaires are heads of companies in Silicon Valley and San Francisco.
If the proposal gets enough signatures, it will appear on the statewide ballot in November.
SEIU-UHW says the tax would help close the gap on federal health care cuts.
"Right now, a massive $100 billion cut is going to hit California's health care system when we roll into 2026," said Saldana.
Supporters say the emergency billionaire tax will prevent a statewide health care collapse.
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"This would help keep ERs open, keep health care workers serving patients," said Saldana.
Congressman Ro Khanna represents much of Silicon Valley.
He supports the proposal, saying it will be good for American innovation.
A spokesperson for Rep. Khanna wrote that the representative has "always supported a modest wealth tax on billionaires to deal with staggering inequality and to make sure people have healthcare."
Governor Gavin Newsom says he opposes the wealth tax proposal.
At the New York Times Dealbook conference earlier in December, he said California has to be pragmatic.
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"It's not something to be panicked about, but it's part of the broader concern and narrative that's developed in this country of the haves and have-nots, not just income inequality, but wealth inequality," said Saldana.
Several tech billionaires have hinted they may leave the Golden State now - in case the proposed wealth tax goes through.
It would retroactively apply to anyone living here on January 1, 2026.
"We've heard big names like Peter Thiel and others that are threatening to leave," said Logan.
White House advisor David Sacks, who is also a tech billionaire investor from San Francisco, posted on x:
'...after blindly funding the Left for years, Silicon Valley is finally realizing what time it is. Dinner Time. And they're on the menu."
San Francisco State University Labor professor John Logan believes that if it happens, only a small number of billionaires would leave.
"Most tech billionaires -- who could easily afford to pay this 5-percent one-off tax-- are not going to upend their lives, move to Austin, Texas, move to Florida, move to other parts of the country, given all the advantages they enjoy," said Logan.