Robert F. Kennedy Jr. hinted strongly on Friday that he would run for president on a third-party ticket instead of continuing his long-shot Democratic primary challenge to President Biden, a move that would set off alarms among Democrats worried about its potential to cause chaos in November 2024.
Mr. Kennedy, in a video released by his campaign, teased a “major announcement” in Philadelphia on Oct. 9, promising to speak about “a sea change in American politics” and dropping clues that he would be continuing his presidential campaign outside the Democratic Party.
“How are we going to win against the established Washington interests?” Mr. Kennedy says in the video. “It’s not through playing the game by the corrupt rules that the corrupt powers and the vested interests have rigged to keep us all in their thrall. Instead, we’re going to have to rewrite the assumptions and change the habits of American politics.”
“What I’ve come to understand after six months of campaigning: There is a path to victory,” he declares at another point, saying that the more he sees the inherent goodness of the American people, “the more the path to victory becomes visible.”
Mr. Kennedy’s top aides declined to elaborate about his intentions. But his supporters have expressed frustration with the Democratic National Committee’s primary process, which has been geared toward backing Mr. Biden’s re-election bid.
“It’s kind of obvious,” said Lincoln Chafee, the former Rhode Island governor and senator, who is backing Mr. Kennedy. “The primaries are so rigged, there’s no debates.”
Mr. Kennedy, he added, “has to look at his options.”
Democrats have watched Mr. Kennedy’s candidacy nervously since it began in April. They fear that any third-party candidacy could siphon off crucial votes from Mr. Biden, ultimately helping former President Donald J. Trump, the current favorite to be the Republican nominee.
Mr. Kennedy, 69, an environmental lawyer and prominent purveyor of conspiracy theories whose family has symbolized Democratic politics for decades, has built a following among Silicon Valley tech executives, disaffected voters in both parties and skeptics of the medical and scientific establishments.
After some polls in the late spring showed him with up to 20 percent of Democratic support, Mr. Kennedy’s fortunes fell as more attention was paid to his panoply of views on the coronavirus pandemic, immigration and vaccines that are well outside the party’s mainstream.
By late summer, surveys showed Mr. Kennedy polling in the low single digits. With his campaign roiled by news coverage of recordings of bigoted remarks he made at a New York dinner, he no longer appeared to be a threat to Mr. Biden.
In recent months, Mr. Kennedy has dropped hints about continuing his campaign as a third-party candidate. He met in July with the Libertarian Party chairwoman and suggested on a podcast that he could leave the Democratic Party.
As the reality set in for Mr. Kennedy that Mr. Biden would not debate him, he began using his platform to become more critical of the party and its presidential election process.
This month, he published an open letter to Jaime Harrison, the D.N.C. chairman, and party members pleading for accommodations.
“The D.N.C. is not supposed to favor one candidate over another,” Mr. Kennedy wrote.
The Biden campaign and its allies at the D.N.C. have summarily dismissed Mr. Kennedy’s candidacy. .
The fact that some Republicans believe it would be advantageous to Mr. Trump if Mr. Kennedy embarks on a third-party run has raised questions about whether anyone in the former president’s world has encouraged it.
But Mr. Kennedy has in recent months become far more popular with Republicans than he is with Democrats. His campaign in the Democratic primary has been supported by Republicans: David Sacks, a donor for Gov. Ron DeSantis of Florida, hosted a fund-raiser for him in June.
While Democrats are nervous that any third-party options would hurt Mr. Biden, it’s not clear from whom Mr. Kennedy would draw more votes if he qualified for the ballot in key battleground states.
Over the years, third-party candidacies have been a focus of Roger J. Stone Jr., Mr. Trump’s longest-serving political adviser.
“I predict #RFK abandons the rigged Democrat nominating process and runs as an Independent,” Mr. Stone wrote on Sept. 24 on X, the website formerly known as Twitter.
In a brief interview on Friday, Mr. Stone said he had no involvement in Mr. Kennedy’s effort. “I’m supporting Donald Trump,” he said.
Corey Lewandowski, an ally of Mr. Trump who served as his campaign manager during the 2016 election, wrote on X in response to an article about Mr. Kennedy’s possible move, “If true the race is over for @JoeBiden!”
Despite Mr. Kennedy’s earlier flirtation with the Libertarian Party, Brian McWilliams, a spokesman for the party, said there had been no recent conversations between Mr. Kennedy and its leadership.
Placing himself on the ballot as a candidate of a newly established third party would be an onerous and expensive proposition for Mr. Kennedy, who would have to navigate ballot access laws in enough states to be a serious presidential candidate.
Maggie Haberman contributed reporting.