The fight to repeal California’s controversial new mandatory vaccine law officially ended this week before it even got to the ballot box, as opponents on Wednesday conceded an ill-fated petition drive fell woefully short on signatures.

But while the latest setback is forcing one of the state’s most vocal and embittered activist groups to acknowledge an organizational breakdown and lack of resources, parents and politicians behind the cause refuse to give up the larger battle: overturning — or, at least, weakening — a law they say violates their parental rights.

Tim Donnelly, the former state assemblyman who led the referendum, acknowledged Wednesday that many Californians weren’t ready to donate to the cause — or provide their signature.

“People are terrified of this issue,” said Donnelly, who ran for governor last year. “The politicians, they want it to go away.”

Senate Bill 277, passed by the Legislature and signed by Gov. Jerry Brown in June, eliminated parents’ rights to refuse to vaccinate their children for personal or religious reasons. It requires that children receive a strict course of 10 immunizations to attend school or day care in the Golden State.

In reaction to a measles outbreak last year, state Sen. Richard Pan, D-Sacramento, led the charge for the tougher requirements despite an uproar from opponents who showed up in force to fight the bill in a series of contentious legislative hearings.

The group’s petition drive required 365,880 signatures by Thursday to qualify a referendum to repeal the law for the November 2016 ballot, a target Donnelly conceded they would miss by a wide margin. In the end, 233,758 signatures had been received by the California Secretary of State.

A lack of funding — the group’s GoFundMe site raised only $170,000 — and plenty of infighting contributed to the petition drive’s failure, Donnelly said, adding the issue was a tough sell, especially among donors and elected officials.

Yet, despite a recent Public Policy Institute of California poll that showed 67 percent of California adults and 65 percent of public school parents say children should not be allowed to attend public schools unless they are vaccinated, the law’s adversaries remain undaunted.

“We don’t want to give up our medical rights like this — it’s a slippery slope,” said Brandy Vaughan, a Marin County resident and founder of the Council for Vaccine Safety. Just days after Brown signed the bill into law on June 30, she organized an anti-SB 277 rally on the Golden Gate Bridge attended by hundreds of protesters.

The former sales representative for Merck said her nonprofit group aims to raise awareness of vaccine risks through public education. But Vaughan, who helped gather signatures for the doomed referendum, acknowledges that opponents “need to be better organized.”

Opponents have already announced a new initiative effort that seeks to restore parental rights on medical and educational decisions.

Meanwhile, other anti-SB 277 grassroots efforts are moving forward with various strategies their leaders say should raise more money to advocate for their goals of destroying, or at least denting, the law.

Political experts say as long as they have the money, they can try — and try again.

“It does not mean they are going to succeed,” said Sherry Bebitch Jeffe, a University of Southern California political expert who noted that getting that initiative on the ballot would require more paid signature gatherers than were hired during the referendum, which relied largely on the work of volunteers.

But the ballot box is not their sole focus.

“Our political strategy around SB 277 is that we recognized that we would probably get more bang for the buck by doing other things than the referendum,” said Christina Hildebrand, a Palo Alto mother of two who founded A Voice For Choice, a nonprofit that addresses the health effects of food and pharmaceutical products.

Hildebrand did not sign a petitition related to the referendum effort. She said the massive amount of money needed to run such a campaign made it an uphill battle.

Instead, she said A Voice For Choice is fundraising to help pay for, among other things, educating and marketing their concerns to the public.

As their website exhorts: “What is your child or your grandchild’s health and education worth? Put a price to it and DONATE THAT AMOUNT NOW to Repeal SB277!”

Donors are encouraged to help the group raise over $350,000. Part of that money is already paying for veteran Sacramento lobbyist Roxanne Gould, whom the group hired in late spring to meet with legislators on key committees. This fall, she and Hildebrand plan to consult with about two dozen lawmakers to pitch an amendment that they say would “clean up” SB 277 by removing two of the 10 shots now required by the state of California — tetanus and hepatitis B — both of which they say are not communicable diseases.

“Countries like Sweden and Germany and a few other European countries that have much more reduced vaccine schedules are not seeing serious outbreaks of things,” she said.

On its website, however, the U.S. Centers for Disease Control explains that people can become infected with the hepatitis B virus during activities such as birth (spread from a mother to her baby during birth) and sex with an infected partner. And while it says tetanus does not spread from person to person, the CDC lists potentially serious health complications for those who don’t receive vaccine protection for both infections.

Hildebrand and others also expect parents will file multiple lawsuits around the state, but they can’t turn to the courts before July 1, when SB 277 goes into effect.

Under the law, only a medical exemption from a physician can exempt a child from the shots; absent that, the child cannot be sent to a public or private school or day care. But the child can be home-schooled, and participate in independent study programs offered by public schools.

Those suits, Hildebrand suggested, will be aimed at school districts around the state that prevent children from going to school, which she argues is a child’s “fundamental right under the California constitution.”

Legal experts, however, have refuted that notion, saying the law does not give unvaccinated children the right to attend school or daycare if their presence places others in jeopardy, including those who cannot be vaccinated.

Other parents may sue alleging the new law violates their religious freedom guaranteed under federal law. But those cases have been rejected by the courts as well, said Dorit Rubinstein Reiss, a professor and vaccine law expert at UC Hastings College of Law in San Francisco.

In fact, Reiss noted, just this week the U.S. Supreme Court said it will not hear a challenge to New York state’s requirement that almost all children be vaccinated before they can attend public school. That decision upheld an appeals court ruling that said the policy doesn’t violate a student’s constitutional right of religious freedom.

“It’s not clear that their strategy will be successful in stopping the law,” Reiss said of SB 277 opponents, adding, “I would be very, very surprised if a court did not uphold the law.”

For his part, Pan, who is facing a recall over the vaccine law, said he is confident most Californians are unswayed by the fight against vaccines.

“It’s unfortunate that so many of them have been made fearful of vaccines from misinformation,” said Pan, a practicing pediatrician. “I think the vast majority of Californians understand why this is so important.”

Contact Tracy Seipel at 408-920-5343. Follow her at Twitter.com/taseipel.