California’s billionaires just poured $40 million into blocking a wealth tax that threatens to force them to pay 5% of their net worth

California’s billionaires are going on the defensive, putting millions of dollars behind an effort to defeat a one-time billionaire tax that would collect 5% of their net worth if approved.

A pair of billionaires, along with other wealthy individuals, have recently upped their contributions to Building a Better California, a PAC formed earlier this year to oppose Proposition 40, which would impose a one-time 5% tax on California residents with more than $1 billion in assets to increase healthcare funding in the state.

Venture capitalist John Doerr put in $7.5 million to the group, while the executive chair of blockchain company Ripple, Chris Larsen, contributed an extra $10 million to the group, according to a campaign finance filing from August 14, the Financial Times reported.

Doerr is worth about $22.6 billion according to Forbes, while Larsen is worth $11.4 billion.

Some multimillionaires also contributed, including the co-founder of cybersecurity company Lookout, John Hering, and Greenoaks Capital founder Neil Mehta, who contributed $946,000 and $250,000, respectively.

The newest contributions come as Building a Better California boasts an endowment of $110 million as of late June—a sign that wealthy Californians are taking seriously the threat the one-time tax represents for their finances if approved by voters in November.

Still, a recent poll by UC Berkeley’s Institute of Governmental Studies shows voters are split on whether to approve the billionaire tax.

The survey of more than 4,000 registered voters found that 48% of likely voters support the measure, while 41% oppose it.

Though registered Democrats overwhelmingly said they would back the proposed tax, 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 fighting propositions Building a Better California isn’t taking any chances.

The group has backed two of its own initiatives, Proposition 41 and Proposition 42 , which would cancel out the billionaire tax if either receives more votes than the billionaires tax, even if the billionaire tax is also approved.

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