(CNN) — President Joe Biden will receive the formal endorsement of more than a dozen members of the extended Kennedy family on Thursday, according to the Biden campaign, aiming to harness the legacy of a storied Democratic family while implicitly underscoring their near-universal rejection of a third-party challenge mounted by one of their own, Robert F. Kennedy Jr.

The broader Kennedy family has mostly shunned RFK Jr.’s campaign, calling it “dangerous,” even as the candidate himself looks to capitalize on his last name and family history.

But Thursday’s event in Philadelphia nonetheless highlights the threat Kennedy poses to Biden as he seeks to win reelection in a race where even a low-performing third-party candidate could act as a spoiler.

In a speech introducing Biden, Kerry Kennedy — one of RFK Jr.’s sisters who has helped lead her family’s organizing efforts — will declare the president “a champion for all the rights and freedoms that my father and uncles stood for” while also warning of threats posed by Donald Trump to democracy and freedom.

“Donald Trump is running to take us backwards, attacking the most basic rights and freedoms that are core to who we are as Americans,” she’ll say, according to excerpts of her speech shared by the Biden campaign. “He has said he will be a dictator on day one, even saying he wants to try to suspend the Constitution.”

“I can only imagine how Donald Trump’s outrageous lies and behavior would have horrified my father, Robert F. Kennedy, who proudly served as Attorney General of the United States, and honored his pledge to uphold the law and protect the country,” Kennedy plans to say.

Biden’s reelection campaign, she’ll say, should “unite not only Democrats, but all Americans, including Republicans, and independents.”

She and Biden will be joined by John White Jr., a civil rights leader in Philadelphia, who will discuss how the Kennedy family affected his public service work.

After the speech, the Kennedys will join Biden at a grassroots organizing event with campaign volunteers and local supporters, where he’ll be introduced by Joe Kennedy III. Family members plan to make campaign phone calls and knock on doors afterward.

It’s not the first time Biden has sought to emphasize his ties to the Kennedys. He has cited Robert F. Kennedy Sr. as a personal inspiration in his political career. A bust of the former attorney general and senator sits in the Oval Office.

He has appointed three family members to diplomatic posts in his administration: Caroline Kennedy as ambassador to Australia; Victoria Reggie Kennedy, the widow of the late Sen. Ted Kennedy, as ambassador to Austria; and Joe Kennedy III as special envoy to Northern Ireland.

On St. Patrick’s Day this year, the extended clan was invited for a reception at the White House and posed for photographs with Biden in the Rose Garden. And Biden invited Maria Shriver, a niece of John F. Kennedy and Robert F. Kennedy, to attend his State of the Union address to highlight her work on women’s health.

In public statements, some members of the Kennedy family have already made clear they are backing Biden in November. Many have sought to distance themselves from RFK Jr.’s presidential bid.

In a CNN interview last month, another sister of RFK Jr., Rory Kennedy, said she’s concerned he could undermine Biden’s chances of defeating Trump.

“I feel strongly that this is the most important election of our lifetime and there’s so much at stake. And I do think it’s gonna come down to a handful of votes in a handful of states, and I do worry that Bobby just taking some percentage of votes from Biden could shift the election and lead to Trump’s election,” she said.

RFK Jr. has sought to downplay an apparent family rift.

“I have a big family, about 105 cousins on the last time we counted,” he told CNN’s Erin Burnett earlier this month. “I have a big family. I don’t know anybody in America who’s got a family who agrees with him on everything.”

The Kennedy family members attending Biden’s event Thursday include Rory Kennedy, Kerry Kennedy, Joe Kennedy II, Beth Kennedy, Joe Kennedy III, Christopher Kennedy, Maxwell Taylor Kennedy, Vicki Strauss Kennedy, Kathleen Kennedy Townsend, Kerry Kennedy Meltzer, Max Meltzer, Ted Kennedy Jr., Stephen Kennedy Smith, Peter McKelvy and Rebeca McKelvy.

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