Sacramento, California - A proposed emergency tax on California billionaires to help fund the state's cash-strapped healthcare system has gathered enough signatures to trigger a referendum in November, the union behind the measure announced.

The proposal gathered 1.5 million signatures – nearly twice the number required to get on the ballot.

"Most Californians and most billionaires recognize how reasonable and necessary this proposal is – both to keep emergency rooms open and to save California businesses from closing," Suzanne Jimenez of the Services Employees International Union-United Healthcare Workers West (SEIU-UHW), the measure's lead sponsor, said late Sunday.

Under the California Billionaire Tax Act, the state's wealthiest residents would be hit with a one-time tax of 5% of their net worth.

Some 90% of the tens of billions of dollars in expected revenue would be used to fund the state healthcare system for five years to offset the massive federal cuts imposed by President Donald Trump's budget law.

While the tax would be a one-off, the proposal has sparked controversy in the most populous state and across the nation.

Opponents fear it will scare off Silicon Valley and trigger an exodus of the ultra-wealthy, which they say would hurt tax revenue. California is home to more than 250 billionaires, more than any other state.

Governor Gavin Newsom, a Democrat who is considered a potential 2028 presidential hopeful, opposes the tax, while progressive Senator Bernie Sanders is its chief supporter in Congress.

High-profile entrepreneurs are against it. According to US media, Google co-founder Larry Page, Oracle's Larry Ellison, and Palantir co-founder Peter Thiel have all taken steps to reduce their footprint in California.

And the cryptocurrency and artificial intelligence communities are funding ad campaigns against it.

Jimenez, the union leader, described the measure's wealthy opponents as "a very small group of the most controversial billionaires on the planet."

"Healthcare workers and our allies won't quit until we fully protect our patients from the looming healthcare disaster that will be caused by $100 billion in cuts to California healthcare," she added.