WASHINGTON >> A broad coalition of the Kennedy family endorsed President Joe Biden today at a campaign rally in Philadelphia, pointedly rejecting one of their own in Robert F. Kennedy Jr., the independent candidate who many Democrats believe poses a significant threat to Biden’s reelection chances.

Among the relatives of Kennedy who backed Biden were his siblings Joseph, Kerry, Rory, Kathleen, Maxwell and Christopher. Roughly a dozen Kennedys appeared at the rally, where Kerry Kennedy introduced Biden.

“He has us thriving again, believing again, behaving like good neighbors again,” Kerry Kennedy said of Biden as he and five of her siblings looked on from the stage. “Nearly every single grandchild of Joe and Rose Kennedy supports Joe Biden. That’s right, the Kennedy family endorses Joe Biden for president.”

Accepting the endorsement, Biden said that Robert F. Kennedy’s “passion and courage inspired my generation,” recounting his speech calling for peace after the assassination of the Rev. Martin Luther King Jr.

The show of force by the Kennedys sent the clearest signal yet that America’s most storied Democratic family is deeply fearful that one of its own could tip the 2024 election to former President Donald Trump, and hopes to use its influence to try to stop him.

Many family members have previously expressed strong disapproval of Robert F. Kennedy Jr.’s candidacy, voicing anguish about his promotion of conspiracy theories and confusion about why he is challenging a Democratic president they admire. Like many Democrats, they worry that he could help Trump win if he draws even a small number of votes away from Biden in the battleground states — contests that were decided by tens of thousands of votes in 2020.

Responding to the move by his family members, Kennedy refrained from criticizing them.

“I hear some of my family will be endorsing President Biden today,” he wrote on social media. “I am pleased they are politically active — it’s a family tradition. We are divided in our opinions but united in our love for each other.”

Trump remained the main target of attacks at the Philadelphia event, with Kerry Kennedy calling him “the most anti-democratic president in American history.”

Still, the symbolic repudiation of Kennedy was not subtle. His sister said she believed Biden’s campaign “should unite not only Democrats but all Americans, including Republicans and independents,” groups her brother is hoping to appeal to. She repeatedly emphasized the need for unity to defeat Trump.

A person familiar with the matter, who insisted on anonymity, said that Kennedy family members had approached the Biden team and requested a joint event for the endorsements.

The family had telegraphed its intentions: Last month, members visited Biden at the White House for St. Patrick’s Day, sharing a photo of him with a large group of them. “From one proud Irish family to another — it was good to have you all back at the White House,” Biden wrote on social media.

Members of the Kennedy family also denounced an ad that a super political action committee supporting Kennedy ran during the Super Bowl. The ad closely resembled a spot supporting John F. Kennedy, his uncle, during his 1960 bid for the White House.

With the election months away, and Kennedy still pursuing access to the ballot in many states, it’s hard to know whether he would draw more votes from Trump or Biden’s camp. But polls suggest that Trump’s base of support is much more fixed than Biden’s, meaning it’s possible that some of the president’s voters could be open to an alternative.

The Democratic Party has put together a team of lawyers aimed at tracking Kennedy’s threat, especially in battleground states. The group is seeking to counter other potential spoilers such as Cornel West, a progressive academic seeking the presidency, and the Green Party.

Democrats have already watched the collapse of one third-party effort they had nervously eyed: The centrist group No Labels, after seeking to set up a moderate politician with national recognition as an alternative to Biden and Trump, announced this month that it would abandon its attempt.

Kennedy holds a smorgasbord of policy positions not easily categorized by ideology. He has expressed liberal views on abortion, the environment and income inequality, but has also promoted false theories about the safety of vaccines and pushed arguments that are more common on the right.

This month, he questioned the Justice Department’s effort to prosecute those who rioted at the Capitol on Jan. 6, 2021, downplaying the severity of the attack. His campaign also fired a consultant who had suggested that supporting Kennedy would help Trump defeat Biden.

Still, Trump has signaled that he, too, sees Kennedy as potentially attracting voters away from his campaign. He posted on social media last month that Kennedy was more “radical Left” than Biden, casting him as a liberal Democrat in disguise. Yet Trump has also privately expressed intrigue with the idea of choosing Kennedy as his running mate — a notion that those close to him consider unrealistic and that Kennedy himself rejected.

Biden’s rally in Philadelphia is the final major stop in a three-day swing through Pennsylvania. During the trip, he laid out his economic and tax agenda, repeatedly attacking Trump as wealthy, out of touch and an enemy of working people.

This article originally appeared in The New York Times.

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