California voters will decide on Nov. 3 whether to approve Proposition 40, which would impose a one-time 5% tax on the assets of billionaires living in the state at the start of the year, mainly to fund health care. The roughly 200 billionaires affected could pay in a lump sum or over five years.
Building a Better California, the best-funded opposition group, has raised more than $118 million from 10 donors, more than half of it from Google co-founder Sergey Brin, who has spent over $100 million on ads against the measure. The group is also backing two competing measures that would cancel the billionaire tax if either receives more votes.

Gov. Gavin Newsom, Democratic gubernatorial candidate Xavier Becerra and groups including Planned Parenthood Affiliates of California and the California Teachers Association oppose the tax, arguing it could push the state's biggest taxpayers to leave. The California Democratic Party, the California Federation of Labor Unions and Sen. Bernie Sanders support it.
A Public Policy Institute of California poll in May found 54% of likely voters backing the measure.