Who has more backbone with California’s Democratic Party rank and file—two-term Golden State Gov. Gavin Newsom, or socialist Vermont Sen. Bernie Sanders? Apparently the answer is the man backing the wealth tax, as the state party on Sunday endorsed the measure for the November ballot.

Mr. Newsom has come out against the supposedly one-time 5% tax on the net worth of California residents with more than $1 billion in wealth, on grounds it would cause top earners to flee. He has been joined by the California Teachers Association and a firefighters union, which have raised concerns about losing revenue for schools and other programs. Democratic gubernatorial nominee Xavier Becerra also opposes the measure. The warning is familiar: Leave, and the public must accept less.

On the other side is SEIU Healthcare Workers West, which spearheaded the ballot measure and claims it would raise $100 billion. Ninety percent of the revenue would have to go to health care, including the Medicaid program that provides health care to millions of Californians. Mr. Sanders and the Teamsters support the measure. The proposal would require billionaires to contribute more toward the public systems that support California’s economy, while the party continues to debate whether the ultra-rich should pay more.

Energy and opinion among Democrats are on the right side of this fight. A Public Policy Institute of California poll in May found that 54% of likely voters supported the initiative, including 76% of Democrats. The people have spoken; it is the governor who is not listening.

Proponents argue that federal health-care cuts threaten California’s health-care system and that the wealth tax would provide funding for hospitals, emergency rooms, and Medicaid. Billionaires can afford to contribute more to keeping health care available, even as the figures cited for the program show California’s federal Medicaid payments increased 14%, or $16.4 billion, over the last year.

Health care is among the industries in the state adding jobs, and public investment remains central to the debate over how those services are funded. The wealth-tax fight is also a fight over the party’s tax priorities. If the measure passes, supporters will have a major opening to make a wealth tax part of the party’s 2028 national platform. That is the overdue correction the country needs.

Mr. Newsom has endorsed a national wealth tax after opposing the ballot initiative. But in joining the fight at the last minute, he deserves no credit for the movement he opposed until it was already advancing.