Washington DC - Robert F. Kennedy Jr, scion of America's storied political clan, suspended his long-shot presidential bid on Friday and endorsed Donald Trump, injecting a new dose of uncertainty into the White House race.

"I no longer believe that I have a realistic path of electoral victory," Kennedy, an anti-vaccine activist and conspiracy theorist who was polling in the low single digits, said at a press conference in swing state Arizona.

Kennedy (70) condemned the selection of Vice President Kamala Harris as the Democratic standard-bearer without a primary contest and cited a long list of grievances against his former party that he said had led him to now "throw my support to President Trump."

Kennedy's independent candidacy featured a number of bizarre stories – including about suffering from a parasitic brain worm and depositing a dead bear cub in Central Park – and drew the opposition of most of his famous family, who threw their weight behind the Democratic ticket.

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"Our brother Bobby's decision to endorse Trump today is a betrayal of the values that our father and our family hold most dear," his sister, human rights activist Kerry Kennedy, said on X.

"It is a sad ending to a sad story," said the statement, also signed by four other siblings.

Kennedy's withdrawal came a day after the surging Harris gave an electrifying speech in Chicago, accepting the Democratic presidential nomination and embarking on the final 10-week sprint to election day.

Analysts are mixed on the effect Kennedy's exit will have on the presidential race and how much of his support will gravitate to Trump or Harris. However, in a very tight contest, it is possible that even a few thousand votes in a crucial swing state could determine who wins the White House.

Harris and Trump are neck and neck in the polls less than three weeks before their September 10 debate in Philadelphia.

Harris, a 59-year-old former senator and prosecutor, left the Democratic convention in Chicago with momentum, having outraised Trump and erased the polling leads he enjoyed before she replaced President Joe Biden on the Democratic ticket last month.

But she is not resting on her laurels.

"On to tomorrow," she told NBC News after the speech. "That was good – and now we've got to move on."

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Dan Kanninen, battleground director of the Harris campaign, cautioned that the race remains "very, very tight."

Potential headwinds for Harris include internal party tensions over US policy on the Israel-Hamas war and fallout from Kennedy's withdrawal.

Democratic heavy hitters, from Michelle Obama to Bill Clinton, warned that the party could still lose to Trump's Republicans if complacency creeps in.

"If we see a bad poll – and we will – we've got to put down that phone and do something," the former first lady told the party faithful in Chicago.