Opponents of Proposition 40, California's one-time 5 per cent tax on billionaires, have spent more than $304 million against it before the 3 November vote, a Capital & Main analysis of state records through 20 October found. The analysis was published on 22 October; the total is that outlet's tally, and this publication has not rebuilt it from the filings.

The fight over the tax accounts for nearly half of all money spent on California's ballot measures this year, by the same analysis. Measured against what the tax is meant to raise, it is small. That gap explains both why the money is being spent and why the outcome is hard to call.

It would charge each California billionaire a single tax equal to 5 per cent of net worth. According to the Legislative Analyst's Office analysis in the official voter guide, the tax applies to people who were California residents on 1 January 2026, is due in 2027, and can be spread over five years, at extra cost to the taxpayer. Real estate, pensions and retirement accounts are generally excluded. The analyst's office puts the number of people affected at "a few hundred"; CalMatters put it at roughly 200.