Business groups such as the California Chamber of Commerce, the California Teachers Assn. and Planned Parenthood Affiliates of California oppose the proposal.

Gov. Gavin Newsom and Democrat Xavier Becerra and Republican Steve Hilton, who are running to replace Newsom, also have spoken out against it.

Opponents describe the wealth tax as a “flawed experiment” that will harm California’s economy because companies and rich people will flee the state.

“If they leave, it will create a huge hole in our budget, threatening core services like education, healthcare and public safety,” arguments against the proposition say.

Opponents of the measure say the proposal is an ineffective attempt to address the long-term effects of the healthcare cuts and would harm California’s economy and budget.

The state budget in California already is largely dependent on income taxes paid by its highest earners. Because of that, revenues are prone to volatility, hinging on capital gains from investments, bonuses to executives and windfalls from new stock offerings, and are notoriously difficult for the state to predict.

Billionaires such as Palmer Luckey, the co-founder of defense tech startup Anduril Industries, and entrepreneur Mark Cuban have pushed back against the proposal on social media. Some tech executives have been relocating or moving their assets to try to avoid the potential tax. Google co-founder Sergey Brin has been cutting ties to California, relocating to Nevada while funding groups such as Building a Better California that are backing ballot measures that could nullify the billionaire tax.

More than $187 million has been spent opposing the proposition, with the bulk coming from Building a Better California, cryptocurrency company Ripple Labs and its co-founder Chris Larsen, data from the secretary of state show.