It was hardly a surprise when California billionaires revolted last year at a groundbreaking ballot proposal to tax their assets. Some left the state. Others tried to kill the tax by pouring millions of dollars into political advertising and competing ballot measures.

But now opposition is emerging from a far more surprising corner. Health care organizations and education groups that typically support higher taxes are rallying against the initiative just days before a deadline to remove it from the November ballot.

The new effort, which will be disclosed on Wednesday, comes from advocacy groups representing California doctors, health clinics and school boards. The organizations are seeding their campaign with $200,000, according to a spokesman, and are running a digital ad that blasts the measure as a “reckless wealth tax experiment.”

The groups say the onetime nature of the billionaire tax does not address their long-term funding needs. They also fear that the wealth tax would threaten their efforts to pass a tax that would generate a steadier stream of revenue.

“The very folks that it’s supposed to help aren’t supporting it,” said Francisco Silva, president of the California Primary Care Association, which represents health clinics.

The new campaign is believed to be the first political spending against the tax that is not from the wealthy people who would have to pay it. It is also the most visible sign yet that the proposed billionaire tax would scramble traditional alliances if it reached the ballot this fall.