Jack Ciattarelli, the Republican candidate for governor of New Jersey, stood in front of a debate audience on Sunday and said that he supported childhood vaccines and the state’s role in expanding the number of children who are immunized.

Four days later, Mr. Ciattarelli was a featured guest at an event hosted by a group that has emerged as one of New Jersey’s main opponents of mandatory vaccination. The keynote speaker was a doctor who has falsely claimed that the Covid-19 vaccine magnetizes patients. (“They can put a key on their forehead. It sticks,” the doctor, Sherri Tenpenny, testified in 2021 in Ohio. “They can put spoons and forks all over them and they can stick.”)

Mr. Ciattarelli’s appearance at the New Jersey Public Health Innovation Political Action Committee’s Health Freedom Gala in Hazlet, N.J., on Thursday came as President Trump’s health secretary, Robert F. Kennedy Jr., has elevated immunization skeptics and limited access to Covid vaccines nationwide. On Monday, in an extraordinary White House address, Mr. Trump also warned pregnant women that Tylenol has been linked to autism in claims unsubstantiated by science.

Mr. Ciattarelli’s appearance also came as a new independent poll showed the governor’s race tightening.

Representative Mikie Sherrill, the Democratic nominee for governor, said Mr. Ciattarelli’s appearance at the gala showed “we can’t trust him to keep New Jersey kids safe and healthy.”

“Jack is taking his cues from fringe activists, conspiracy theorists and the likes of Donald Trump and R.F.K. Jr., whose extreme policies will result in reduced access to vaccines and more of our children catching preventable illnesses,” Ms. Sherrill said in a statement.

She made a point in Sunday’s debate of stressing the crucial role that vaccinations have played in eradicating diseases in the United States.

“New Jersey parents should be very worried,” she told the audience, citing a growing number of measles cases in schools.

“These are eminently curable diseases, and we are allowing children to get sick and, yes, die because we are not appropriately following medical research, vaccine protocol,” she added.

New Jersey’s health commissioner reported this year that the state had lost its so-called herd immunity from measles — the immunization threshold needed to protect a community from rampant disease spread. In one of the state’s Republican strongholds, Ocean County, vaccination rates have fallen to 85 percent, the lowest in the state.

On Sunday, Mr. Ciattarelli, who is running for governor for the third time and has been endorsed by Mr. Trump, said, “I support the vaccine schedule.”

“We’ve got to get above the threshold of herd immunity to keep our community safe,” he added. “The obligation of any governor on Day 1, after they take their oath of office, is the public health and safety.”

Earlier this month, however, in answering a question at a town hall, he said: “I would never mandate a vaccine. That’s up to you.”

Thursday’s event, which featured a roughly 15-minute speech by Mr. Ciattarelli, was closed to reporters. He entered through a side door of the catering hall and exited the same way, walking briskly to a waiting car and refusing to answer questions.

The $200-a-plate dinner is an annual event for a group whose stated mission is to protect the rights of New Jersey residents “to make all medical decisions for themselves and their families without coercion or undue constraint.”

Dr. Tenpenny was signing copies of her new book, as women in full-length gowns and men in formal wear filtered past a registration desk topped with a bucket of stress balls stamped “mandate health freedom.”

“It’s not their role to protect us from ourselves,” one guest, Roy Bryan, said of politicians. “Once you’ve got that, you have no more liberty.”

Vaccination mandates have proved to be politically potent in New Jersey — even before the pandemic intensified the debate over immunizations.

In January 2020, New Jersey lawmakers were poised to pass what would have been the nation’s broadest ban on religious exemptions to childhood vaccines. A similar but less sweeping measure had earlier been adopted in New York amid a large measles outbreak in Brooklyn.

Opponents of the legislation in New Jersey flooded the halls of the State House. Del Bigtree, an anti-vaccine activist from Texas, flew in to lead a daylong protest. Mr. Kennedy showed up, too.

After weeks of sustained protest, the legislation collapsed.

Less than a year later, the Covid-19 vaccine would be administered for the first time.

Most New Jersey residents celebrated the vaccine’s power to slow the deadly virus in a state that recorded more than 36,000 Covid-related fatalities. But some skeptics revolted against the scores of executive orders that the governor, Philip D. Murphy, signed in the name of public health.

When Mr. Ciattarelli ran against Mr. Murphy in 2021, pandemic-related mandates tied to masks and vaccines helped to drive Republicans to the polls. Mr. Ciattarelli lost to Mr. Murphy by a narrow margin. Stephen M. Sweeney, the powerful Senate president who was one of the most vocal supporters of the ban on religious exemptions, lost his seat to an underfunded political novice.

Mr. Ciattarelli was interviewed by leaders of the Public Health Innovation PAC during the 2021 campaign.

“I’ve always been about bodily autonomy,” he said, according to the group’s website. “I don’t think government has any right to tell any individuals that they have to take a vaccine or a medicine.”

Attendees were reminded that Thursday’s dinner was not “just a celebration.”

“It’s a rally point for the 2025 election cycle,” organizers wrote. “Together we’ll build momentum, strengthen our coalition and show New Jersey that families are ready for leadership that defends our rights.”