It’s been almost two weeks since San Francisco voters passed Proposition C, and the expected threat of a legal challenge to it — potentially bankrolled by the business interests who opposed the measure — has yet to materialize.

There’s a reason for that: It might not have to.

Prop. C, which seeks to tax the city’s biggest businesses to fund services for the homeless, is already facing an existential threat that bloomed back in August.

That’s when the Howard Jarvis Taxpayers Association sued the city over another ballot measure — also titled Proposition C — that sought to bring in about $146 million a year for child care services and early education by raising taxes on commercial rents. The taxpayers association, business groups and commercial property owners say the city violated the state Constitution when it allowed the child care measure to pass with a simple majority vote, rather than a two-thirds majority.

Since the mid-1990s, any ballot measure that would raise taxes for a specific purpose has required a two-thirds majority to pass. But last year, the San Francisco City Attorney’s Office issued a memo interpreting a recent state Supreme Court ruling to mean that tax measures put on the ballot by citizens needed only a simple majority to pass. The threshold is still two-thirds for tax measures put on the ballot by government officials.

To date, San Francisco remains the only known city in the state to knock down the voter threshold for citizen-driven tax measures.

“San Francisco is alone in this,” said Michael Coleman, a California municipal finance expert who works as the principal fiscal policy adviser both to the California Society of Municipal Finance Officers and the League of California Cities. “There’s no other city attorney’s office that I know of that has come up with a similar opinion.”

Since San Francisco’s June election, three ballot measures — the homelessness and child care initiatives and June’s Proposition G, which sought to raise teachers’ wages with a $298 parcel tax — have passed, each reaching more than 50 percent of the vote. City Attorney Dennis Herrera went on the offensive in September, asking the courts to validate the use of a simple-majority threshold to pass Prop. G. Any decision could affect the two Prop. Cs as well.

The city will begin collecting the tax revenue mandated by each of the three initiatives, but won’t spend that money until the legal questions about the voter thresholds are resolved. That will probably take years, given the expectation among legal experts that the cases would probably be appealed to the state Supreme Court.

For Herrera and his office, the stakes are high. Up to $500 million a year in funding for child care, homelessness and teacher salaries now depends on whether the courts agree with his singular reading of the state Supreme Court’s intentions in what’s known as the City of Upland decision.

The San Bernardino County case doesn’t touch explicitly on citizen-driven tax measures. Instead, the court ruled more broadly that citizens aren’t limited by the same constraints as government officials when putting measures on the ballot.

A ruling against the city in one case could endanger all three propositions. But if the courts agree with Herrera, it could bode well for their survival.

“San Francisco has always been a leader,” Herrera said in a statement sent by a spokesman. “We have some of the sharpest legal minds and most dedicated people in the country. When the California Supreme Court spoke, we examined that decision and provided our clients with the best legal advice we could. This is about defending the will of the voters,” he said.

“The court placed great weight on the difference between taxes emanating from a legislative body and taxes emanating from an initiative petition,” said John Coté, a spokesman for the City Attorney’s Office. “The same reasoning should apply whether the issue is the type of election or the vote threshold needed.”

But in his survey of the state, Coleman said other attorneys and advocacy groups like the League of California Cities “generally do not read it the way San Francisco’s counsel has read it. They don’t believe the Upland measure paves the way.

“I’m not saying that’s bad or wrong, but San Francisco tends to be out there testing the edges of the bounds of things. That’s the case here,” he said.

In an effort to invalidate the child care measure, the Jarvis taxpayers’ association suit argues San Francisco violated the state Constitution when it allowed the initiative to pass with a simple majority. Beyond that, it also maintains that the measure, placed on the ballot through a signature-gathering drive, was actually a stalking horse for a government-led goal.

Four supervisors — Jane Kim, Norman Yee, Sandy Lee Fewer and Hillary Ronen — originally joined together in an effort to put the measure on the ballot themselves, as city law permits. Passing the measure that way would have required a two-thirds majority because it was put on the ballot by the Board of Supervisors. But they withdrew their signatures in favor of a signature-gathering effort.

“Government action is government action however it’s brought to the table,” said Laura Murray, a staff attorney for the Jarvis association and a lead lawyer in the case against the city. The case, Murray said, demonstrates what her group claims to be one of the most corrosive consequences of Herrera’s memo: the potential for city officials to use the lower, simple-majority threshold to launder government-led tax measures through the initiative process.

“This case makes it so clear what the potential is for collusion between city officials and citizens’ groups,” Murray said.

Yee insisted that the child care measure was “driven by the community in the first place.”

He and the other three supervisors, he said, put the measure on the ballot themselves “as a backup, thinking that, if the community can’t figure it out, we have this alternative,” he said. “At the end of the day, (the community) figured it out and they got the signatures. We didn’t want to leave any options off the table.”

Proponents of the three ballot measures and outside observers say they’re confident that, with Herrera and his team at the helm, the money that voters approved will be spent as planned — eventually.

“The San Francisco City Attorney’s Office is outstanding,” said Darien Shanske, a professor at UC Davis School of Law who specializes in state and local taxation. “For those who like the Upland decision and would like to see its logic (around voter thresholds) developed, having the San Francisco city attorney taking the lead is about as positive as it can be.”

Dominic Fracassa is a San Francisco Chronicle staff writer. Email: dfracassa@sfchronicle.com Twitter: @dominicfracassa