WASHINGTON — Robert F. Kennedy Jr said Monday he was quitting the race for the Democratic US presidential nomination to run as an independent -- a move that could shake up a tight election by siphoning votes from the main candidates.

A rematch between President Joe Biden and his predecessor Donald Trump is by far the most likely scenario in 2024, and a third-party hopeful taking even a few votes from Biden in swing states could spell disaster for the Democrat.

Many analysts however believe that Kennedy, a long-time anti-vaccine conspiracy theorist and a darling of the right-wing US media, would harm Trump more than Biden.

"I've come here today to declare our independence from the tyranny of corruption which robs us of affordable lives, our belief in the future and our respect for each other," the son of Robert F. Kennedy and nephew of John F. Kennedy told supporters in Philadelphia.

"But to do that, I must first declare my own independence: Independence from the Democratic Party, from all other political parties."

The 69-year-old former environmental lawyer has long identified as a Democrat, and invoked his assassinated father in an anecdote about the 1968 campaign trail.

"I haven't made this decision lightly. It's very painful for me to let go of the party of my uncles, my father, my grandfather and both of my great-grandfathers," he added.

Kennedy was introduced by his wife of nine years, "Curb Your Enthusiasm" star Cheryl Hines, who said her husband was ready to make the country better for all Americans and "bring them together."

But he has proved more popular among Republicans than Democrats in opinion polling -- he is languishing on an average of less than 15 percent in major primary polls, 47 points behind Biden.

Embraced by conspiracy theorists on the far right such as Alex Jones and Trump's former national security advisor Michael Flynn, Kennedy has claimed that AIDS might not be caused by HIV and that wifi causes cancer and "leaky brain."

He has blamed antidepressants for school shootings and said chemicals in tap water could make children become transgender.

He ignited a firestorm of criticism in July over claims that Covid-19 was "ethnically targeted" at Caucasians and Black people, while Ashkenazi Jews and the Chinese were spared.

Last week, liberal academic Cornel West ended his Green Party candidacy in favor of an independent run. Many Biden allies see the 70-year-old African American philosopher as a bigger threat than Kennedy.

Four of Kennedy's siblings nevertheless released a statement calling his announcement "dangerous" and "deeply saddening."

"Bobby might share the same name as our father, but he does not share the same values, vision or judgment," they said.

Republicans called Kennedy a "Democrat in Independent's clothing."

"He is your typical elitist liberal and voters won't be fooled," Republican National Committee chairwoman Ronna McDaniel said in a statement.

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