Support for California’s proposed billionaire wealth tax has weakened ahead of the November election, with likely voters narrowly divided over Proposition 40, according to a new statewide poll. The survey found that 45% of likely voters would support the measure, while 43% would oppose it and 12% remained undecided. Support had led in an earlier February poll, but the latest results show the initiative facing a difficult campaign as opponents expand their advertising efforts. Proposition 40 would impose a one-time tax of 5% on the assets of California’s wealthiest residents. The proposal has become one of the most closely watched fights in state politics, with Gov. Gavin Newsom and major interest groups campaigning against it. Jack Citrin, a University of California, Berkeley political science professor and co-director of the poll, said the measure’s standing had shifted in favor of opponents. “From a historical point of view, you’d think the no side are gaining and would potentially win,” Citrin said. The campaign is likely to become substantially more expensive. Supporters have received nearly all of their funding from the health care union SEIU-UHW, while billionaire opponents have committed tens of millions of dollars to defeat Proposition 40 and advance competing initiatives that would nullify it. That financial imbalance could shape how voters understand the proposal. The poll found that support was weaker among people who doubted the tax would truly be limited to a single payment. Opponents recently began airing an advertisement warning that Sacramento would seek another tax after the initial revenue was spent. Jon Cohen, founder and CEO of TrueDot, the firm that conducted the poll, said the measure was vulnerable because its support was not firmly held. “The fact that the people who are the most tuned in are the most opposed points to significant vulnerability,” Cohen said. The survey also identified potential advantages for supporters. Among undecided likely voters, 65% said they leaned toward voting yes, compared with 15% who leaned no. The measure received support from 66% of Democratic likely voters. Across party lines, 44% of likely voters said billionaires do more harm than good, while 34% viewed them favorably. Still, the poll suggested that the key contest could be for moderate and independent voters. Citrin said those voters may be more receptive to arguments about economic risks and the reliability of the tax’s one-time structure than to appeals centered primarily on taxing extreme wealth. About 83% of likely voters said they had heard at least some information about Proposition 40, including a substantial share who said they had heard “a great deal.” Among that most-informed group, opposition exceeded support by 57% to 38%. The fight could also affect Newsom’s national political prospects if he seeks the Democratic presidential nomination in 2028. A plurality of registered voters, including a plurality of Democrats, said they believed Newsom opposed the tax to protect billionaires and other wealthy interests rather than to protect California’s economy and tax base. Cohen characterized that finding as an issue-specific credibility problem rather than evidence of a broader collapse in support for Newsom. But by making himself a prominent opponent of Proposition 40, the governor is taking a position that conflicts with many voters in his own party and with widespread public skepticism toward billionaire wealth.