Campaign over Prop 40, as measure was known, was most expensive in state history, flush with Silicon Valley dollars California billionaires won a contentious ballot measure campaign, defeating a proposed one-time tax on their wealth. The bruising campaign over Proposition 40 was the most expensive in state history and witness to back-and-forth dealing from tech billionaires and democratic leaders, including Gavin Newsom, the governor, who steadfastly opposed the measure. While supporters of Prop 40 cast it as a solution to drastic healthcare cuts from Trump’s One Big Beautiful Bill Act, opponents led by prosperous billionaires mounted an influential campaign saying the billionaire tax was instead “an everyone tax”.