Groups backed by some of the world’s richest people are pouring cash into a final push against California’s proposed wealth tax, escalating what’s already one of the country’s most expensive battles of this election cycle.

Building a Better California, a political influence operation bankrolled by Sergey Brin and other billionaires, has raised about $318 million since it was created earlier this year, including $21 million in donations disclosed on Saturday. Most of the money has gone toward efforts to defeat Proposition 40, a ballot measure that would impose a one-time 5% tax on California billionaires.

The group expects to reach around $350 million in total fundraising by Election Day, a person familiar with its efforts said, asking not to be named discussing private conversations. A spokesperson for Building a Better California declined to comment.

The spending is intensifying a fight that has divided California’s Democratic establishment and prompted vast political donations by billionaires, turning the measure into a test of voters’ appetite for taxing extreme wealth. With less than a month to Election Day and polls showing a close race, opponents are using their overwhelming financial advantage to flood television and digital platforms with ads attacking the measure.

Most of Building a Better California’s financial power has come from Brin and other Silicon Valley billionaires. The Google co-founder has so far donated $102 million to the group, which is also backing ballot measures unrelated to the wealth tax. Other supporters include venture capitalist John Doerr, former Google CEO Eric Schmidt and Ripple co-founder Chris Larsen.

There’s more advertising queued up. Proposition 40 opponents have spent over $73 million on ads so far, including reservations of future ad space, according to AdImpact.

Wealth tax opponents have also looked beyond Silicon Valley for contributions. They have approached wealthy potential donors in the Los Angeles area and its surrounding region, people familiar with their strategy said.

So far, the main Building a Better California contributor outside technology circles is Stewart Resnick, chairman of agricultural giant The Wonderful Co., filings show. Saturday’s disclosure included a donation from Jim Coulter, founding partner and executive chair of TPG Inc.

The wealth-tax fight has already surpassed some of California’s costliest ballot battles of recent years, including campaigns over rent control, redistricting and defining Uber drivers as independent contractors. It still falls short of the roughly $460 million spent on sports betting initiatives in 2022.

“This may set a record and, in California, where those records are so high, that would be eye-opening,” said Thad Kousser, a professor of political science at the University of California at San Diego. “I feel like their pockets are almost infinitely deep.”But while billionaires are financing the majority of the campaign, they are steering clear from appearing on the ads. Instead, ads running across cable, streaming and digital platforms highlight other opposition groups.

“Teachers, firefighters, carpenters and Planned Parenthood Affiliates of California all have this in common: They all oppose the so-called wealth tax,” a narrator says in one of the ads. Another features politicians opposed to the measure, including Governor Gavin Newsom. A third one that began running in recent weeks shows a woman claiming the wealth tax has a “hidden loophole” that could allow politicians to tax her “savings account, 401(k), even my home equity.”That strategy plays into the inherent advantage of campaigns that are trying to defeat ballot measures, said Brandon Castillo, a veteran California ballot strategist.

“On the ‘No’ side, you just have to create skepticism even with one small part of the measure,” he said.

Supporters of Proposition 40 are airing their first television ad, which features Vermont Senator Bernie Sanders. They have also tried countering the ad barrage by highlighting divisions within opposition groups, including Planned Parenthood.

As the campaign heads into the final stretch, the competition for voters’ attention across California will drive up the cost of advertising, said Castillo.

“The last three weeks is when it becomes most intense,” Castillo said. “It’ll just get cluttered because all the other campaigns will be piling in, and it will cost more to break through.”