California's billionaires are going on the defensive, pouring millions of dollars into an effort to defeat a one-time wealth tax that would collect 5% of their net worth if approved by voters.

Two billionaires, together with other wealthy individuals, have recently increased their contributions to Building a Better California, a political action committee formed earlier this year to oppose Proposition 40. The ballot measure would impose a one-time 5% tax on California residents with more than $1 billion in assets, with the revenue directed toward increasing healthcare funding in the state. The levy would fall on accumulated net worth — a one-time charge on holdings rather than a recurring tax on annual income.

Venture capitalist John Doerr put in $7.5 million, while Chris Larsen, executive chair of blockchain company Ripple, contributed an additional $10 million, according to a campaign finance filing from August 14, the Financial Times reported. Forbes estimates Doerr's fortune at about $22.6 billion; Larsen's stands at $11.4 billion.

Several multimillionaires also contributed, including John Hering, co-founder of cybersecurity company Lookout, who gave $946,000, and Neil Mehta, founder of Greenoaks Capital, who gave $250,000. The contributions span venture capital, blockchain, cybersecurity and investment firms — the industries that dominate the ranks of the measure's named wealthy opponents.

The new donations come as Building a Better California reports an endowment of $110 million as of late June — a sign that wealthy Californians are taking seriously the threat the one-time tax poses to their finances ahead of the November vote. The committee has also reserved $87 million worth of advertising time ahead of the election to sway public opinion, the New York Times reported last month.

Voters, however, remain split. A recent poll by UC Berkeley's Institute of Governmental Studies surveying more than 4,000 registered voters found that 48% of likely voters support the billionaire tax — short of an outright majority — while 41% oppose it. Registered Democrats overwhelmingly said they would back the measure, but only 50% of unaffiliated voters said the same, while 80% of Republicans said they would not support the proposal.

"These results suggest that the Billionaires Tax initiative is shaping up to be a closely fought contest, with the key question being whether opponents can make big enough inroads among the state's traditionally Democratic-leaning voters," said Eric Schickler, co-director of the Institute of Governmental Studies, in a press release.

Two counter-initiatives on the ballot

Building a Better California is not taking any chances. The group has backed two initiatives of its own, Proposition 41 and Proposition 42, which would cancel out the billionaire tax if either receives more votes than Proposition 40 — even if the wealth tax itself is also approved.

Proposition 41 would require the state auditor to review any special tax proposal before it is presented to voters. Proposition 42 would ban new taxes based on the mere ownership of assets such as property, which are usually taxed only when sold.

It is unclear whether either counter-initiative has a better chance of passing than the billionaire tax. The Institute of Governmental Studies survey found that while 72% of voters had heard of the billionaire tax, fewer than a third of the state's voters were aware the two counter-initiatives existed — a gap that, combined with the thinner support among unaffiliated voters, frames the persuasion fight in the campaign's final stretch before the November vote.

Politicians divided

The measure has also split the state's political class. Governor Gavin Newsom and Xavier Becerra, the Democratic candidate for governor, have come out against the billionaire tax. But earlier this month, the California Democratic Party endorsed the proposal, dealing a blow to billionaire opponents of the bill.

Among those opponents, Google co-founder Sergey Brin is among the most adamant. The world's fourth-richest man moved many of his assets out of California late last year and has now put $102 million toward opposing the California wealth tax, following an additional $20 million contribution he made to Building a Better California earlier this month.

Other billionaires, including former Shark Tank star Mark Cuban, have also come out against the billionaire tax. In an exchange on X over the weekend, Cuban warned Representative Ro Khanna (D-Calif.) that the tax would hurt entrepreneurs and innovation in the state.

"IMO, if this passes, only idiot startup founders stay in Cali," Cuban wrote in a post.

Prominent politicians outside the state, by contrast, have pushed for the billionaire tax to pass. Senator Bernie Sanders of Vermont said in February that the billionaire tax would help show the wealthiest Americans "we are still living in a democratic society where the people have some power."

In March, Sanders and Khanna introduced legislation that would take a version of California's billionaire tax to the national level, effectively using the state ballot fight as a template for federal policy. Their bill, the "Make Billionaires Pay Their Fair Share Act," would establish a 5% wealth tax on America's 938 billionaires to expand Medicare, reverse cuts to Medicaid made by President Trump's Big Beautiful Bill, and provide a $3,000 direct payment to every man, woman, and child in households making $150,000 or less.