Robert F. Kennedy Jr. Makes His White House Run Official

For Mr. Kennedy, that cause is vaccine skepticism, which he cloaked in terms of truth-seeking and free speech, a crusade that in the past led him to falsely link childhood vaccines to autism. At the height of the Covid-19 pandemic, he sought to undermine public trust in vaccines, comparing government efforts to impose mandates in some places to “Hitler’s Germany.” Both Facebook and Instagram took down accounts of a group he runs for spreading medical misinformation.

Family members have accused Mr. Kennedy of sowing distrust in the science behind vaccines. His campaign has appalled members of his famous Democratic clan.

“I love my brother Bobby, but I do not share or endorse his opinions on many issues, including the Covid pandemic, vaccinations and the role of social media platforms in policing false information,” Kerry Kennedy, a sister of Mr. Kennedy’s, said in a statement.

Bob Shrum, a former aide to Ted Kennedy, said Mr. Kennedy’s attacks on Dr. Anthony S. Fauci and the federal government’s top medical and scientific agencies would have infuriated his uncle. “It’s contrary to everything his uncle Ted Kennedy ever did,” he said. “He called health care the cause of his life.”

In a statement of his own, Mr. Kennedy said he had made a “difficult choice to put my principles ahead of my personal affections” for Mr. Biden. “Some members of my family agree with me and others do not,” he added. “I bear them no ill will.”

On Wednesday in Boston, Mr. Kennedy was introduced by his wife, Cheryl Ruth Hines, an actress, and he pointed out his children and grandchildren seated in a balcony of the hotel ballroom. But there was a notable absence of members of his family who have made careers in politics. A number of them affirmed their support for Mr. Biden’s re-election in recent days, part of a decades-long alliance between the Biden and Kennedy families.

Ted Kennedy’s widow, Victoria Reggie Kennedy, is Mr. Biden’s ambassador to Austria. Caroline Kennedy, the daughter of President Kennedy, is ambassador to Australia. And Joe Kennedy III, a grandson of Robert F. Kennedy, is Mr. Biden’s special envoy to Northern Ireland, who flew with him this month to Belfast.