Robert Kennedy Jr, a noted anti-vaccine advocate whose uncle "JFK" was the 35th US president, formally announced his own White House bid on Wednesday.

The 69-year-old outsider intends to compete for the Democratic Party's nomination against incumbent President Joe Biden, who has yet to announce his reelection bid but has repeatedly said he plans to run.

Kennedy's father "RFK" served as attorney general under Democratic president John F Kennedy and later as a US senator from New York.

The assassinations of both brothers – JFK in 1963 and RFK in 1968 during his own presidential campaign – are viewed as among the most significant events in 20th century US politics.

RFK Jr worked for decades as an environmental lawyer, but has been known since 2005 for promoting conspiracy theories about vaccines, notably linking them to the development of autism.

"I've come here today to announce my candidacy for the Democratic nomination for president of the United States," he said at his campaign launch event in Massachusetts, the stronghold of the Kennedy family.

Among the crowd assembled in Boston, several supporters wore caps with anti-vaccine messages.

Kennedy vowed to end the deep political division in the United States by "telling the truth to the American people."

Despite his highly recognizable name, Kennedy's bid is a longshot against incumbent Biden, whose own official launch is expected later in the year, though no date has been announced.

A few others have also announced they will challenge 80-year-old Biden for the Democratic nomination, but no high-profile names have joined the race. (AFP)