California’s proposed Proposition 40 would impose a one-time 5% tax on the net worth of hundreds of California-based billionaires if approved in the November midterms. Forbes estimates that the state’s 10 richest billionaires alone could owe a combined $31.5 billion, with Meta CEO Mark Zuckerberg facing the largest bill. The estimates may differ from final liabilities because California’s tax rules could treat trusts and private-company valuations differently from Forbes’ methodology. Forbes values Anthropic CEO Dario Amodei and his sister, cofounder and president Daniela Amodei, at about $13 billion each, implying roughly $680 million in tax apiece; a public listing could raise their estimated fortunes above $20 billion and their potential bills to at least $1 billion each. Support for the measure has weakened, with a UC Berkeley-Los Angeles Times poll showing 50% of likely voters opposed, 42% in favor and 8% undecided, compared with a narrow majority supporting it six weeks earlier. Propositions 41 and 42 could also limit or prohibit wealth taxes; among Propositions 40, 41 and 42, the measure with the most yes votes would pass, potentially nullifying Proposition 40 even if it wins a majority. A Stanford University working paper published by the National Bureau of Economic Research found that lost income-tax revenue from billionaires leaving California could outweigh the one-time proceeds, with 85% of 100,000 modeled outcomes producing negative revenue. The proposal could affect professional sports owners depending on residency and asset valuations, including Los Angeles Chargers majority owner Dean Spanos, Joe Lacob and Peter Guber of the Golden State Warriors, José E. Feliciano and Kwanza Jones of the San Diego Padres, Hasso Plattner of the San Jose Sharks and Henry Samueli of the Anaheim Ducks. Other owners, including Steve Ballmer, Charles Johnson, Philip Anschutz, Denise DeBartolo York and John York, appear to live outside California and may not be affected. More than $404 million has been donated to campaigns on both sides, while opponents including Eric Schmidt and Sergey Brin have spent millions fighting the measure.