Some of California's wealthiest technology and investment figures are pouring extraordinary sums into campaigns that could defeat a proposed one-time tax on billionaires, turning Proposition 40 into one of the most expensive and consequential ballot fights of the November election.

Billionaire-backed political groups have raised more than $150 million for efforts opposing the tax directly or supporting rival measures that could prevent it from taking effect.

Google co-founder Sergey Brin has emerged as the dominant financier, contributing $102 million to Building a Better California.

Proposition 40 would impose a one-time tax equal to 5% of net worth on billionaires who were California residents on January 1, 2026. The tax would become due in 2027, although taxpayers could spread payments over five years at an additional cost.

Real estate, pensions and retirement accounts would generally be excluded.

Ninety percent of the proceeds would have to finance public healthcare, while the remainder would go to education, food assistance and administration of the tax.

The amount the measure would actually raise is one of the central disputes.

Supporters, led by Service Employees International Union-United Healthcare Workers West, have promoted estimates of roughly $100 billion over several years as California faces reductions in federal healthcare and safety-net funding.

The state's nonpartisan Legislative Analyst's Office is considerably more cautious, saying collections would probably total tens of billions of dollars and that the exact amount is particularly difficult to predict because fortunes fluctuate and wealthy taxpayers can change their behaviour.

Brin's financing is significantly larger than that of other individual opponents.

Peter Thiel is also involved in the anti-tax effort, but descriptions presenting Thiel and Brin as roughly equal leaders of a combined $100 million campaign are misleading.

Brin alone has supplied more than $100 million to Building a Better California.

Thiel has contributed millions to a California Business Roundtable political committee involved in the broader opposition effort.

Crypto entrepreneur Chris Larsen and Ripple Labs have also contributed millions, while venture capitalist John Doerr has added substantial funding to Building a Better California.

The money is financing more than conventional advertisements urging voters to reject Proposition 40. Building a Better California is also backing Propositions 41 and 42, two separate November measures whose provisions could conflict with the billionaire tax.

The Legislative Analyst's Office says that if either Proposition 41 or Proposition 42 receives more yes votes than Proposition 40, courts could prevent Proposition 40 from taking effect even if a majority of voters approves it.

Proposition 42 would prohibit new state taxes on ownership of financial assets and other personal property, directly challenging the legal foundation of the proposed wealth tax.

The advertising battle has become increasingly aggressive.

One business-backed campaign portrays Proposition 40 as an 'everyone tax' and warns that ordinary Californians could eventually face similar levies.

The proposal actually placed on the ballot, however, applies specifically to billionaires who met the California residency test on January 1, 2026. It does not automatically extend the 5% levy to ordinary households.

Arguments that the measure could establish a political precedent for broader taxation are predictions about future policy, not provisions of Proposition 40 itself.

Opponents are also emphasizing the possibility that wealthy residents will leave California and take future income-tax revenue with them.

Several billionaires altered their residency before the January 1 cutoff, and opponents have cited approximately $500 billion in combined wealth associated with prominent departures.

That figure should not be confused with $500 billion in lost state tax revenue.

California's Legislative Analyst estimates that behavioural responses to Proposition 40, including billionaires leaving the state, would probably reduce ongoing state income-tax revenue by less than $1 billion annually.

The political divide does not follow a simple party line.

Democratic Governor Gavin Newsom opposes Proposition 40, arguing that a state-level wealth tax risks driving major taxpayers and investment elsewhere and that taxation of extreme wealth should instead be addressed nationally.

The California Democratic Party nevertheless endorsed the measure in August, with its executive board narrowly clearing the 60% threshold required for endorsement.

Other opponents include several labor and healthcare organisations, while prominent progressive politicians support the tax.

Supporters are also raising substantial money, although far less than the broader billionaire-backed campaign network.

California campaign-finance records list more than $31 million in contributions from entities associated with SEIU-United Healthcare Workers West to the principal committee supporting Proposition 40. That makes descriptions of the pro-tax campaign as spending only a few million dollars increasingly outdated as the November contest intensifies.

For now, voters remain closely divided.

An August Berkeley Institute of Governmental Studies poll found 48% of likely voters supporting Proposition 40 and 41% opposing it, a seven-point advantage that still leaves a sizeable undecided group.

The measure will be decided in California's November 3 general election alongside the rival tax measures that could determine whether it survives even if voters approve it.