Biden, a Democrat, faces Republican Donald Trump in a November re-match of the 2020 election. But members of both parties have bristled over the possibility that the candidacy of Kennedy, a prominent anti-vaccine activist, or another third-party bid could spoil either of their chances.
Kennedy, son of the slain U.S. senator and presidential candidate Robert F. Kennedy, made his name as an environmental lawyer, before becoming known as an anti-vaccine advocate, and with an eclectic mix of political views. He is backed by 15% of registered voters, versus 39% for Biden and 38% for Trump, according to a recent Reuters/Ipsos poll.
Many in Robert F. Kennedy Jr's elite family - prominent Democrats since the 19th century - have broken with him over his views.
"I can only imagine how Donald Trump's outrageous lies and behavior would have horrified my father, Robert F. Kennedy," Robert F. Kennedy Jr's younger sister Kerry Kennedy will say at the Biden campaign event, according to prepared remarks. "Daddy stood for equal justice, human rights, and freedom from want and fear. Just as President Biden does today."
Kerry Kennedy and other members of the family are then expected to join local volunteers in door-knocking and phone-banking on Biden's behalf, campaign aides said.
The event comes as Biden spends his third day this week in Pennsylvania, a critical swing state for his re-election bid. Biden needs strong turnout from Philadelphia's Black community to win the state, and the Kennedys became icons for many African American families because of their advocacy for civil rights.
Biden, only the second Catholic president after John F. Kennedy, has long spoken of how he was inspired by the family's political legacy.
In his 2007 book "Promises to Keep," Biden describes himself as a young man moved to get into public service by the Kennedy brothers and the late civil rights activist Martin Luther King Jr., "swept up in their eloquence, their conviction, the sheer size of their improbable dreams." A bust of President Kennedy sits in the Oval Office.
When Biden was later elected to Congress, fellow Senator Ted Kennedy would become one of his closest friends. Ted, John and Robert Sr. were brothers.
The family ties have continued in the years since. Dozens of members of the Irish American family joined Biden at the White House for St. Patrick's Day last month.
Caroline Kennedy, daughter of the slain president, was appointed ambassador to Australia by Biden. She had served as ambassador to Japan under Democratic President Barack Obama.
"I have a big family," Robert F. Kennedy Jr told Reuters last month. "Many of them are working in my campaign. Not everybody agrees with me."
(Reporting by Trevor Hunnicutt; Additional reporting by Stephanie Kelly and Jarrett Renshaw; Editing by Mary Milliken and Leslie Adler)
By Trevor Hunnicutt
