Gov. Gavin Newsom said he would fight the proposed billionaire tax.

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Gov. Gavin Newsom's fight to stop a proposed tax on billionaires in California is coming down to the wire.The so-called billionaire tax — which would impose a one-time 5% tax on California residents whose assets exceed $1 billion — could appear on the ballot this November if critics aren't able to stop it ahead of a June 25 deadline.Newsom previously vowed to fight the proposed tax, telling The New York Times in January he believed it would hinder innovation. "I'll do what I have to do to protect the state," he said.The measure was proposed by SEIU-UHW, a healthcare workers union, which announced in April that it had submitted 1.55 million signatures, nearly double the roughly 875,000 needed. The deadline for initiative measures to qualify for the November ballot is June 25.With the deadline approaching, several California groups that generally align with Democrats are coming out publicly against the initiative and pressuring proponents to withdraw the measure.The California Teachers Association — one of the state's largest teachers unions — voted to oppose the tax, saying it would not provide sustainable funding for schools and communities. Planned Parenthood Affiliates of California and several other labor groups have also come out against it.The measure has the support of progressive Rep. Ro Khanna, the labor union Teamsters California, and the California Democratic Socialists of America.