A billionaire civil war is brewing inside Sergey Brin’s $100 million operation to kill California’s proposed billionaire tax as ex-Google CEO Eric Schmidt is among a group of grumbling billionaires that say the coalition has lost the plot, the California Post has learned.

Schmidt, a $3 million donor to Google-founder Brin’s Building a Better California, was reportedly furious after the state Democratic Party endorsed the union-backed wealth tax and shot down two counter-measures the group is bankrolling.

“Eric is rightfully upset,” a source with knowledge of the matter told The Post.

Some donors and advisers are growing frustrated that Building a Better California, which raised a fortune from Silicon Valley mega-donors including venture capitalist John Doerr with $10 million, Michael Moritz with $7.5 million, is suffering mission creep in its effort to reshape California politics by backing six different state ballot measures.

The group’s original mission was to kill the billionaire tax, viewed as a disastrous policy that would cripple California’s economy by driving wealth out of the state. But some insiders are growing concerned that its spending in other areas, like environmental policy, could threaten its success.

“Once you do that, you’re in a place where you may not be as effective,” one adviser to a Building a Better California donor told The Post, warning about “splintering” the mission.

Schmidt’s frustration only intensified after the bruising defeat two weeks ago at the California Democratic Party’s executive board meeting in San Diego.

The party not only voted to endorse Proposition 40 — the union-backed measure that would impose a one-time 5% tax on the wealth of Californians worth more than $1 billion — but also opposed Propositions 41 and 42, two measures backed by the Brin-funded operation as counterweights to the tax. 

Courtni Pugh, a partner at Hilltop Public Solutions and former director for the Service Employees International Union, had been tasked with lobbying Democrats to remain neutral on the billionaire tax while also whipping support for Props. 41 and 42, according to sources.

Instead, the billionaire coalition walked away 0-for-3, as the billionaire tax narrowly cleared the 60% threshold required for the party endorsement.

Hilltop, which is also working on the gubernatorial campaign of Democratic frontrunner Xavier Becerra, has received $75,000 from the pro-Prop. 42 committee. Becerra and Gov. Gavin Newsom, who also has worked closely with Hilltop on ballot measures, both oppose the billionaire tax.

Pugh did not respond to requests for comment.

Building a Better California raised $118 million through June, with Brin accounting for $82 million of it at the time. He has since thrown down another $20 million to bring his pot commitment to $102 million.

On July 24, the $20 million was split evenly between committees for Props. 41 and 42, giving them a lifeline after both ballot measure committees exhausted nearly $50 million apiece to collect petition signatures and qualify for the November ballot.

The group’s widening ambitions, however, are creating some strange bedfellows — and even putting its donors in uncomfortable positions.

Building a Better California has contributed $10 million to Proposition 45, which would significantly streamline environmental reviews under California’s landmark CEQA law for housing, transportation, water, energy and other projects.

The measure has been pitched by supporters as a way to lower housing and infrastructure costs by cutting red tape.

But Schmidt’s wife, environmental philanthropist Wendy Schmidt, is helping lead the campaign against Prop. 45 with actress Jane Fonda, believing it would gut environmental safeguards.

Along with past and future spending expected on Props. 40, 41, 42 and 45, Building a Better California has sent $6 million to Prop. 37, $1 million to Prop. 1 and millions more to another proposed constitutional tax-and-spending measure.

Other billionaire tax opponents are growing concerned by the group’s tactics.

“This is about not only what are you against,” the source said, “but what are you for?”

Despite some internal disagreement on priorities, more money is expected to flow toward the state propositions in the coming days and weeks.

The New York Times reported that Brin’s group has reserved nearly $87 million of advertising time across the state to air TV commercials.

A source with knowledge of the situation told The Post that the money is being allocated now to save on ad buys before they become more expensive closer to November’s election.

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