Robert F Kennedy Jr, an environmental lawyer, anti-vaccine activist and son of the former senator, is expected to announce Monday in Philadelphia that he is dropping his Democratic Party bid for president and will run as an independent.
Mr Kennedy's campaign flagged the announcement late last month in a video posted to YouTube, promising a "sea change in American politics".
Mr Kennedy's combination of deep-pocketed backers, famous name and lack of enthusiasm for leading Republican candidate Donald Trump or Joe Biden, the Democratic president seeking re-election, could give his campaign a boost, political strategists say.
He could draw the support of about one in seven US voters, according to a Reuters/Ipsos poll released last week, and pull votes from both Mr Biden and Mr Trump.
At a Democratic National Committee gathering in St Louis last week, party faithful were both dismissive and concerned.
"Outside of his name, what does he have? He’s an anathema to his family’s legacy. It’s sort of a disgrace,” said Ken Martin, head of Minnesota’s Democratic Party.
Many Democrats also point to the challenges independent candidates face getting on general election ballots in many states, such as collecting tens of thousands of signatures.
Others expressed caution.
"It’s going to be a very, very close race, so every vote counts. We can’t go into this election with rose-colored glasses,” said Charles Wilson, chair of Washington DC’s Democratic Party.
