Robert F. Kennedy Jr. to Run for President as Independent, Leaving Democratic Primary

In a move that could alter the dynamics of the 2024 election, Robert F. Kennedy Jr. said on Monday that he would continue his presidential run as an independent candidate, ending his long-shot pursuit of the Democratic nomination against an incumbent president.

“I am here to declare myself an independent candidate for president of the United States,” Mr. Kennedy told a crowd of supporters outside the National Constitution Center in Philadelphia.

Since announcing his candidacy in April, Mr. Kennedy, 69, has been a sharp critic of Democratic leadership, which he has accused of “hijacking the party machinery” to stifle his challenge to President Biden. He has also said, in interviews and in public appearances, that the party has abandoned its principles and become corrupted.

Running as an independent will entail an expensive, uphill battle to get on the ballot in all 50 states. Last week, Cornel West, a liberal academic and presidential candidate, said he would run as an independent, abandoning his efforts to secure the Green Party’s nomination.

The scion of a liberal political dynasty, Mr. Kennedy has alienated family members and many Democrats with his promotion of conspiracy theories, his rejection of scientific orthodoxies and his embrace of far-right political figures.

Mr. Kennedy, an environmental lawyer, has been lionized by a movement that has expanded beyond anti-vaccine sentiments, including opposition to the mandatory vaccination of children, to push back more broadly against state public health measures. In recent years, his open suspicions about the government’s handling of the coronavirus and his criticism of lockdowns and vaccine policies gave him a new platform and earned him popularity among many Americans who wearied of the pandemic.

As a candidate, he has built a base of support made up of disaffected voters across the political spectrum, but some Democrats have worried he poses the biggest threat to their party, fearing that any third-party candidacy could peel off voters from Mr. Biden.

Shortly after Mr. Kennedy entered the race, some polls showed him with up to 20 percent of Democratic support — which was in large part a measure of the desire among some for an alternative to Mr. Biden. Mr. Kennedy’s numbers have sagged in recent months, though his campaign, which dwells as much on nostalgia for his political lineage as it does on skepticism about the scientific and political establishment — continues to appeal to a particular cross-section of skeptical Democrats, political conservatives and independents.

Mr. Kennedy has raised two main complaints about the Democratic National Committee, which is supporting Mr. Biden’s re-election effort. First, he said, Mr. Biden and the party pushed to change the first primary state from New Hampshire — where Mr. Kennedy, who has New England roots, enjoys a base of support — to South Carolina, the state that rescued Mr. Biden’s campaign in 2020.

Second, the party has refused to arrange for debates between Mr. Biden and Mr. Kennedy. Mr. Biden’s campaign and the D.N.C. have also essentially refused to acknowledge Mr. Kennedy’s candidacy, and have avoided saying his name.

“It should be a party’s voters who choose a candidate, not party insiders who anoint one,” Mr. Kennedy wrote in a letter to the D.N.C. last month.

“The DNC is not supposed to favor one candidate over another,” he added. “It is supposed to oversee a fair, democratic selection process, and then support the candidate that its voters choose.”

Mr. Kennedy had teased Monday’s announcement in a video last week, though his campaign held off on confirming that he was changing parties. But in the hours before he got on stage in Philadelphia, there was a subtle change on Mr. Kennedy’s campaign website. Where it had once read “I am a Kennedy Democrat,” with the family name in italics, it was changed to: “I am a Kennedy American.”