California voters next month will decide on whether to tax billionaire wealth, a proposal designed to generate billions of dollars to shore up the state’s healthcare system. But Stanford economists are now cautioning that the tax could backfire, leaving the state with less in tax revenue than before. 

Seven billionaires have already publicly moved out of California, removing about $540 billion, or 30%, of the state’s total billionaire wealth from potential taxation, according to Stanford University economists Benjamin Jaros and Joshua Rauh in a new working paper published by the National Bureau of Economic Research. The billionaires who have left the state include venture capitalist Peter Thiel and Google co-founders Larry Page and Sergey Brin.

Because additional billionaires have likely quietly moved since the tax proposal was announced, the measure may ultimately generate only about $30 billion, far less than the $100 billion estimated by its proponents, the economists said. If enacted, the tax would apply a one-time 5% state tax on billionaire wealth. 

To be sure, $30 billion is still a sizable amount of new tax revenue. But future income tax losses from those billionaire departures could more than offset that windfall, the economists calculated.

California could lose roughly $1 billion to $3.5 billion a year in income tax revenue due to its shrinking billionaire population, the paper’s authors estimated. If those losses persist permanently, their value in today’s dollars would exceed the wealth tax proceeds by about $39 billion, the economists said.

“These findings suggest that the Billionaire Tax Act will not fulfill the primary functions for which it was designed,” Jaros and Rauh wrote. “Although the Act was designed to raise revenue, behavioral response prior to voting has eroded most of the projected tax base.”

The Service Employees International Union-United Healthcare Workers West (SEIU-UHW), a healthcare worker union that introduced the ballot measure, didn’t immediately respond to a request for comment on the new research.

Other economists have come to different conclusions about the tax. Economists including Daron Acemoglu of MIT and Joseph Stiglitz of Columbia University, both Nobel Prize winners, have signed their support for the measure, pointing out that California has attracted more venture funding since the wealth tax was announced. 

“What will allow California to thrive in the future is not letting a handful of billionaires live tax free: it is adequate public spending on health, education and public infrastructure, key engines of economic growth to which it is only fair to ask the ultra-wealthy to contribute,” they wrote in a September letter of support for the tax. 

Ahead of the vote on the ballot measure, called Proposition 40, voters in California are apparently divided on the tax. About 46% of likely voters say they favor Proposition 40, while 43% are opposed, according to a new poll from Citrin-Politico.

Dave Regan, the leader of the union that introduced the measure, told CBS News’ Jo Ling Kent that he’s “pretty confident” the tax will pass. 

“There’s never been a proposal like this anywhere in the world, so it’s a novel idea,” Regan said. “We are facing an emergency.”