California voters are narrowly backing a proposed one-time tax on billionaires’ assets, even as two competing ballot measures intended to block it show slightly stronger support, according to a new Public Policy Institute of California poll. Proposition 40, which would impose a 5% tax on billionaires’ assets, is supported by 52% of surveyed voters. The measure’s lead remains fragile as opponents prepare a major advertising campaign ahead of the election. Propositions 41 and 42 are part of a strategy to nullify Proposition 40 if it passes. Proposition 41 would require audits of state spending and invalidate the wealth tax based on how its revenue would be distributed. Proposition 42 would prohibit taxes on financial assets, including stocks and other investments. The poll found 51% support for Proposition 41 and 54% support for Proposition 42. Under California’s ballot rules, if conflicting initiatives both pass, the measure receiving more votes takes effect. The campaigns against Proposition 40 have a substantial financial advantage. A group backed by Google co-founder Sergey Brin has reserved about $100 million in television and other advertising time. Since the measures qualified for the ballot in late July, the group has spent $41.5 million opposing Proposition 40 and $32 million supporting Propositions 41 and 42. The campaign for the wealth tax is being led by organized labor. Its leader, Dave Regan, is also facing allegations of misconduct that he strongly denies, adding another complication for supporters of the measure. A separate labor-backed initiative, Proposition 3, has a clearer standing in the poll. The measure would make permanent a 2012 income-tax increase on California’s highest earners and is supported by 58% of voters. The competing tax measures could shape more than state revenue policy. Their outcome may determine whether California can impose a new levy on the wealthiest residents and how billions of dollars in potential revenue are allocated, while also testing the influence of large donors in the state’s direct-democracy system.