It was a holiday party at a crypto titan's estate in Marin County, and Sergey Brin had a bone to pick with Gavin Newsom.

Mr. Brin, a Google co-founder and one of the world's richest people, is a longtime friend of Mr. Newsom, the California governor. Both men attended each other's weddings. But now Mr. Brin pulled Mr. Newsom aside to a different part of the property for a serious talk.

Mr. Brin told Mr. Newsom that he could not stand the state's proposed billionaire tax. They were soon joined by Mr. Brin's girlfriend, Gerelyn Gilbert-Soto, a Trump-loving gut-health influencer. Even as she tried to defuse the tension — joking that she would let Mr. Newsom's bad policies slide because he was handsome — she argued that the measure would wreck California's economy.

Mr. Newsom, who had never seemed inclined to support the tax, came out the next month and pledged to defeat it. He declined to comment on the interaction.

The December confrontation, which took place at a party thrown by the billionaire Chris Larsen and was recounted by three people briefed on it, reflected Mr. Brin's new war footing. He is growing more politically agitated, more willing to spend his estimated $273 billion fortune on elections and evidently more receptive to Republican points of view.

Mr. Brin, 52, long showed little interest in politics. When he did, he embraced liberal causes: He donated to a campaign to defend same-sex marriage in California in 2008 and backed President Barack Obama's re-election bid in 2012. He called President Trump's election in 2016 "deeply offensive" in leaked comments to Google employees and then joined a protest against Mr. Trump's ban on immigrants from several predominantly Muslim countries. In 2021, he quietly started a nonprofit group that has spent at least $88 million on climate and environmental policy.