WASHINGTON — Robert F. Kennedy Jr. stood before the Lincoln Memorial in January 2022 and condemned the federal government’s coronavirus response by railing against totalitarianism. Jews in Nazi Germany, he suggested, had more freedom than Americans facing vaccination mandates and school, church and business closures in the era of Covid-19.

“Even in Hitler’s Germany, you could cross the Alps to Switzerland,” he told a crowd of flag-waving anti-vaccine enthusiasts at a “Defeat the Mandates” rally. “You could hide in an attic like Anne Frank did.”

Mr. Kennedy later apologized, though it was not the first time he had invoked the Holocaust. Over the past two decades, as he has pursued what he calls “safe vaccine activism,” Mr. Kennedy has evolved from an environmental lawyer concerned about mercury poisoning into a crusader for individual liberty — a path that has landed him, a scion of a storied Democratic clan, in the unlikely embrace of the American political right.

On Wednesday, Mr. Kennedy, plans to formally announce that he is challenging President Biden for the Democratic nomination for president. His vaccine skepticism gives him something in common with another candidate: former President Donald J. Trump, who like Mr. Kennedy has blamed childhood Vaccines for autism — a discredited theory that has been repudiated by more than a dozen peer-reviewed scientific studies in multiple countries.

“Robert F. Kennedy could jump into the Republican primary for president and only DeSantis and Trump, I think, would do better,” Stephen K. Bannon, Mr. Trump’s former chief strategist, said recently on his podcast, referring to Gov. Ron DeSantis of Florida. Mr. Bannon said Mr. Kennedy had a “massive following” with his audience. “People love this guy,” he said.

Vaccination is a singular public health success that has saved untold millions of lives. Vaccines have eradicated smallpox, averted millions of deaths from measles and sent naturally occurring polio cases plummeting, from an estimated 350,000 in 1988 to six reported cases worldwide in 2021, according to the World Health Organization.

Mr. Kennedy has insisted that he is not opposed to vaccines and that his sole interest is in making them safer. “I’m not anti-vaccine, although I’m kind of the poster child for the anti-vax movement,” he said during a recent speech at Hillsdale College, a conservative Christian college in Michigan.

But through his nonprofit, Children’s Health Defense, and his movies, speeches and books — including one that portrays Dr. Anthony S. Fauci as in the pocket of the pharmaceutical industry — Mr. Kennedy has used his platform and his family’s star power to sow doubts about vaccine safety, spreading misinformation by twisting facts out of context.

In 2021, the Center for Countering Digital Hate named him one of its “Disinformation Dozen” — the 12 people whom the organization found to have been responsible for roughly three-quarters of anti-vaccine content on Facebook.

Facebook and Instagram have removed the accounts of Children’s Health Defense, and Mr. Kennedy has accused them of censorship. He is also suing the Biden administration and Dr. Fauci, who for decades led the National Institute of Allergy and Infectious Diseases, accusing them of pressuring social media companies to censor free speech.

Mr. Kennedy declined to be interviewed. In an email message, he said Children’s Health Defense had “an extremely robust fact-checking operation.” He also pointed to a response by Meta, Facebook’s parent company, disputing the “Disinformation Dozen” report. Meta critiqued the study’s design, saying that focusing on just 12 people “misses the forest for the trees.”

Family Backlash

Mr. Kennedy, 69, is the third-eldest child of Robert and Ethel Kennedy and a nephew of former President John F. Kennedy, who urged Americans to take the Salk polio vaccine and signed the Vaccination Assistance Act of 1962 to help states and cities carry out childhood immunization programs.

His activism, and now his political aspirations, have been wrenching for his family. Some of his family members have publicly rebuked him. His sister Rory Kennedy told CNN she was backing Mr. Biden, while his sister Kerry Kennedy said in a statement, “I love my brother Bobby, but I do not share or endorse his opinions on many issues.”

Ahead of his White House bid, Mr. Kennedy is playing up his family history. He lives in California but plans to make his announcement in Boston, a city closely identified with the Kennedys. He recently tweeted a photo of himself in a vintage “Kennedy for President” T-shirt.

His name and family reputation have opened doors for him. Dr. Fauci said he had met with Mr. Kennedy several times and had told him “that I believe that his intentions are not evil, but his information is incorrect, and he’s misguided and can inadvertently cause significant harm.” Dr. Fauci said that when Mr. Kennedy’s book about him, titled “The Real Anthony Fauci,” came out in 2021, he was “really shocked.”

“The entire book is such a complete lie,” Dr. Fauci said.

Mr. Kennedy’s messages often have a grain of truth. The Children’s Health Defense website, for instance, says “vaccines contain many ingredients, some of which are known to be neurotoxic, carcinogenic and cause autoimmunity.” Vaccines do contain preservatives and additives, such as aluminum salts, which have been in use in vaccines for decades. Studies show adverse reactions are rare and typically involve skin allergies.

The Children’s Health Defense website also states that certain vaccines are not tested against placebos in clinical trials, citing polio, hepatitis and meningitis vaccines as examples. That is misleading. Brand-new vaccines — from polio to measles to Covid-19 — are tested in large clinical trials that include placebo groups. But scientists agree it would be unethical to withhold lifesaving vaccines from study participants. For that reason, when older vaccines are reformulated or updated, studies do not include a placebo group.

“Vaccine injuries can and do happen,” the website declares. That is true as well, but the federal government has an aggressive system to track and detect side effects so they can be addressed.

The measles vaccine, for instance, lowers the platelet count in about one in every 25,000 to 30,000 people. That can cause red spots from bleeding under the skin — a problem that is usually “short-lived and self-resolving,” said Dr. Paul A. Offit, the director of the Vaccine Education Center at Children’s Hospital of Philadelphia. But measles causes that problem in one in 3,000 children — roughly 10 times as many as the vaccine, he said.

“There are no risk-free choices, just choices to take different risks,” said Dr. Offit, who has been a vocal critic of Mr. Kennedy. “You could argue the greatest risk of vaccines is driving to the office to get them.”

A Movement Grows

By his own account, Mr. Kennedy was at first a reluctant critic of vaccination. He got involved in 2005, when he was an environmental lawyer suing coal-fired power plants to force them to reduce emissions of mercury and other toxic chemicals.

The anti-vaccine movement in the United States had been growing amid debate over a rise in cases of autism. In 1998, a British doctor named Andrew Wakefield published a study of 12 children in The Lancet, a prestigious medical journal, that suggested a link between the measles, mumps and rubella vaccine and autism.

The article was retracted in 2010, and Mr. Wakefield was later barred from practicing medicine. But in the years after its publication, another theory began to take hold: that thimerosal, a mercury-based preservative that had been used for decades to prevent bacteria from growing in multiple-dose vials of vaccines, caused autism.

The measles, mumps and rubella vaccine never contained thimerosal, but other vaccines given to infants did. The Centers for Disease Control and Prevention says there is “no evidence” that the low doses of thimerosal in vaccines cause harm, “except for minor reactions like redness and swelling at the injection site.”

But in 1999, after Congress directed the Food and Drug Administration to look at mercury in all products, the American Academy of Pediatrics, federal health agencies and vaccine manufacturers agreed that thimerosal should be removed from childhood vaccines. The decision was made “out of an abundance of caution,” said Daniel Salmon, the director of the Institute for Vaccine Safety at the Johns Hopkins Bloomberg School of Public Health.

But the move alarmed parents. As Mr. Kennedy traveled the country giving speeches, he has said, mothers of intellectually disabled children began buttonholing him, pressing him to investigate vaccines.

“They would say to me in a very respectful but also kind of vaguely scolding way, ‘If you’re really interested in mercury exposures to children, you need to look at vaccines,’” he told the Hillsdale College audience.

In 2005, Rolling Stone and Salon copublished an article by Mr. Kennedy, headlined “Deadly Immunity,” that blamed thimerosal in vaccines for fueling the rise in autism. Salon later retracted the article. Mr. Kennedy insisted Salon caved to pressure from government regulators and the pharmaceutical industry.

Thimerosal is still used in flu vaccines. In 2015, shortly after Mr. Kennedy published a book about the preservative, he met Eric Gladen, an engineer who believes he was sickened by thimerosal in a tetanus vaccine and who made a film about his experience. The two joined forces. Mr. Gladen’s advocacy group, World Mercury Project, was later rebranded as Children’s Health Defense.

“We had two huge tools to raise funds; we had my film, which is about 10 years of research put into 90 minutes, and his book,” Mr. Gladen said in an interview, adding, “Between him being a Kennedy, the film and his book, it compelled a lot of people to get involved.”

The anti-vaccine movement was, at the time, largely the province of the political left. Mr. Kennedy found allies in Hollywood celebrities like Jim Carrey and Jenny McCarthy. In California, he waged an unsuccessful fight against a bill to eliminate the “personal belief” exemption that allowed parents to opt out of vaccinating their children.

Mr. Kennedy has been a vocal opponent of the Vaccine Injury Compensation Act, a 1986 federal law intended to promote the development of vaccines by shielding manufacturers from lawsuits. In 2003, at the height of the thimerosal controversy, a bipartisan measure to update the law by offering immunity to vaccine additive manufacturers collapsed in Congress.

Mr. Kennedy points to such efforts as evidence that lawmakers and federal regulators are conspiring to protect drug companies, which he says lack incentives to focus on safety. During the fight over the California legislation, he invoked those arguments, said Dr. Richard Pan, a former state senator who was an author of the bill and met with Mr. Kennedy at the time.

“He mainly focused on the F.D.A. being corrupt and in cahoots with the pharmaceutical companies to hide the danger of vaccines,” Dr. Pan said.

Meeting With Trump

Shortly before Mr. Trump was inaugurated in January 2017, Mr. Kennedy met with him at Trump Tower in Manhattan. Mr. Kennedy said afterward that the president-elect wanted him to lead a “vaccine safety and scientific integrity” commission. He told Science magazine that Mr. Trump had told him he had five friends whose healthy children “developed a suite of deficits” after being vaccinated.

The commission never came to pass, but the coronavirus pandemic gave Mr. Kennedy an even bigger platform. As the country grew ever more polarized, with many of Mr. Trump’s followers shunning the vaccines and Dr. Fauci becoming a lightning rod, Mr. Kennedy’s book about Dr. Fauci became a best seller.

Another book by Mr. Kennedy is due out in June, this time focusing on the controversy over the origins of the coronavirus. Titled “The Wuhan Cover-Up,” it claims that federal health officials “conspired with the Chinese military” to hide the pandemic’s origins — an assertion that appears to conflate experiments by the Chinese military at the Wuhan Institute of Virology with other work there funded by the U.S. government.

How much Mr. Kennedy will talk about vaccine safety during his presidential campaign remains unclear. As he did during the rally at the Lincoln Memorial, he used his talk at Hillsdale College to cloak his activism in a broader point — that the government, the press and social media companies are trying to silence him, pushing the United States toward tyranny.

“The founders, specifically Hamilton, Madison, Adams, said, ‘We put freedom of expression in the First Amendment because all the other amendments are dependent on it,’” Mr. Kennedy said. “Because if you give a government the right to silence their opponents, they now have a license for any atrocity.”