Backers of a ballot measure to divert a portion of San Francisco’s hotel-tax revenue to pay for arts programs and homeless family services watched with dismay last year as their initiative won well over half the vote, yet still lost.

Now they may go back to the ballot — and thanks to a legal opinion by the city attorney, half the vote plus one will be enough.

City Attorney Dennis Herrera’s office told the Elections Department in a legal memo last week that, thanks to a recent state Supreme Court ruling, citizen-instigated ballot initiatives to raise taxes now need only a simple majority to pass instead of two-thirds, the standard that had been in place for more than 20 years.

The finding could help usher in a profound shift in the way in which public projects, like roads, schools or arts programs, are funded in San Francisco.

Ever since state voters passed Proposition 218 in 1996, special taxes — ones intended to fund specific projects — have needed a two-thirds vote to pass. However, in August, the state Supreme Court issued a ruling that appeared to apply that tough standard only to measures put on the ballot by government officials, not ordinary citizens.

The city attorney’s memo said that’s exactly what the ruling did — good news for the backers of the hotel-tax measure, Proposition S, which gained a 63.7 percent “yes” vote in November, and other citizens who qualify a tax measure by gathering signatures on petitions.

The city attorney’s conclusion “certainly means that we should expect to see more initiative-based tax proposals, for good and for bad,” said Alan Auerbach, an economics professor at UC Berkeley and the director of the Robert D. Burch Center for Tax Policy and Public Finance.

The state court’s ruling had touched off a debate among tax policy experts, legal scholars and local officials over whether the justices meant to upend decades of precedent by relaxing the requirement for citizen ballot initiatives. The case itself had to do with whether the city of Upland (San Bernardino County) had violated the rights of the California Cannabis Coalition in 2014 when it refused to put a measure on the ballot that included a tax on marijuana businesses.

Despite lingering uncertainties about the court’s intentions, and the prospect of future lawsuits challenging the ruling, “it seems very likely that voters may now propose special taxes by initiative subject only to a majority vote,” said the memo by Herrera’s office.

“The issue of whether a tax measure could be approved by a simple majority, rather than a two-thirds majority — that was not clear from that (court) opinion,” said John Arntz, director of the city’s Elections Department. “We needed to know what the number is to indicate whether something passed or not, so that’s what the city attorney’s office is providing guidance on.”

Jonathan Moscone , chief of civic engagement at Yerba Buena Center for the Arts and a co-author of Prop. S, said the city attorney’s memo has reinvigorated discussions among the arts community about putting a similar measure in front of voters next year . Prop. S would have diverted slightly more than half of the city’s 14 percent hotel tax to the arts and to programs to help homeless families. All the tax’s revenue now goes to the city’s general fund.

“I’m heartened, and I think anybody would find this heartening,” Moscone said. “It changes the threshold for the city to coalesce around the will of the majority of the people and democratizes the process in a way that gives citizens more of a voice to make change. It’s a powerful time to be a citizen in San Francisco.”

Anti-tax activists and some state Republican lawmakers, however, are chafing at the prospect that the court’s ruling will make it easier to pass new taxes. The Howard Jarvis Taxpayers Association, which represented Upland before the state Supreme Court, has asked the justices to clarify their ruling and has filed a petition to reargue portions of the case. Assemblyman Chad Mayes, R-Yucca Valley (San Bernardino County), has introduced legislation that would ask voters to reinstate the supermajority requirement for special taxes through a constitutional amendment.

“You use the supermajority to ensure greater consensus on really, really important matters,” said Jon Coupal, president of the Howard Jarvis Taxpayers Association. “At the very core of it, this opens the door to additional taxation.”

Requiring only a simple majority to pass citizen-sponsored measures could embolden interest groups, like casino operators, stadium backers or unions, to try to levy new taxes for their own benefit, Coupal said. His group has also raised concerns about local governments doing the same — “colluding” with interest organizations to enact new taxes more easily.

The court will decide by the end of November whether to grant a rehearing on the two-thirds issue.

“The case is not over,” Coupal said.

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