California voters will decide this November whether to impose a one-time wealth tax on the state's richest residents, setting up what could become one of the most expensive ballot measure fights in state history.Proposition 40 would levy a one-time 5% tax on the total wealth — not just income, but stock holdings, real estate, yachts, and other assets — of roughly 200 of California's richest residents. Backers say the measure would raise $100 billion over five years, money they intend to use to offset a wave of federal healthcare funding cuts set in motion by the One Big Beautiful Bill Act, which Congress passed and President Donald Trump signed into law last year.That legislation imposes about $1 trillion in cuts to federal healthcare funding nationwide, the vast majority of it from Medicaid. California is projected to lose more than any other state — $100 billion, according to the RAND Corporation.The ballot measure is the brainchild of Dave Regan, president of the SEIU-United Healthcare Workers West union.California is looking to offset a $100 billion loss in healthcare funding by implementing a one-time wealth tax.Genaro Molina/Los Angeles Times via Getty Images"It would stabilize and backfill the cuts that President Trump's legislation has inflicted on California and on the rest of the country," Regan said.The stakes are high. The federal cuts are expected to roughly double California’s uninsured population, which currently stands at about 2 million people, according to the California Legislative Analyst's Office. Dozens of hospitals could close, and Regan said as many as 150,000 healthcare workers could lose their jobs."This is a pending catastrophe for healthcare in California," Regan said.The proposal has nevertheless drawn opposition from prominent state Democrats, including Gov. Gavin Newsom, along with the California Teachers Association and other groups.Dan Newman, a political consultant who has worked with Newsom in the past, is running a PAC called Stop the Squeeze to defeat the measure. His central argument: Wealthy Californians would simply leave the state, taking their tax dollars with them."They're just vastly underestimating the amount of capital flight that will happen," Newman said, pointing to European countries that have tried and abandoned wealth taxes. "We've seen some examples of it. Sweden and Denmark and Finland and France have all tried and then abandoned this."Sweden and France did repeal wealth taxes after wealthy residents left the country, though Sweden's experience is more complicated than it first appears. The picture elsewhere in Europe is mixed. Denmark is now considering reviving a wealth tax; Spain has raised wealth taxes without triggering a mass exodus of the rich; Norway and Switzerland continue to tax wealth, as well.Newman also echoed Newsom's position that any wealth tax should be imposed at the national level, not state by state, and argues Prop 40 poses a distinct threat to California's startup economy. The measure could force founders to sell off or shutter companies whose paper wealth vastly outstrips their cash on hand, he said — and could push future startups to launch elsewhere."Startup value is often on paper, but only on paper," Newman said. "And suddenly, these folks get a massive bill due for funds that they don't have."Nationally, hostility toward the ultra-wealthy has been building. A recent Wall Street Journal poll found that three-quarters of Americans believe billionaires have too much power in Washington, and a Pew Research Center survey points to some public skepticism toward extreme wealth."If Proposition 40 is going to pass, it's going to be as a result of this extremely enhanced hostility that's developed among the voting public toward the extremely wealthy," said Dan Schnur, who teaches political communications at the University of Southern California and UC Berkeley.Schnur cautioned that passage isn't guaranteed, even in a state as reliably progressive as California. A recent poll found just 49% support for the measure among voters in historically progressive San Francisco — and that was before opposition advertising begins this fall. However, more recent polling shows Prop 40 with a slight majority support statewide.Wealthy tech investors, including Google co-founder Sergey Brin, are funding ad campaigns to defeat the measure. Both sides have raised millions of dollars for their efforts."It will not only be an incredibly vicious fight over Proposition 40, but it may end up being one of the most expensive ballot measures in the history of California politics," Schnur said.If the measure passes, Schnur expects other Democratic-leaning states to pursue similar wealth taxes of their own.