Robert F. Kennedy Jr., scion of America's storied political clan, suspended his long-shot presidential bid on Friday and endorsed Donald Trump, injecting new 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 news conference in swing state Arizona.

Kennedy, 70, condemned the selection of U.S. 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."