WASHINGTON (AP) — President Joe Biden will accept endorsements from at least 15 members of the Kennedy political family during a campaign stop Thursday in Philadelphia as he aims to undermine Donald Trump and marginalize the candidacy of independent Robert F. Kennedy Jr.

Kerry Kennedy, a daughter of former Attorney General Robert F. Kennedy, niece of former President John F. Kennedy and sister of the current presidential candidate, will deliver the endorsements of Biden, according to the Biden campaign.

The decision to highlight the Kennedy family's support more than six months from Election Day is an indication of how seriously Biden's team is taking the threat of a long shot candidate using his last name's lingering Democratic magic to siphon support from the incumbent.

Kennedy Jr. played down the endorsements, writing on social media that his family was "divided in our opinions but united in our love for each other." He said his campaign was about "healing America."

Given Kennedy Jr.'s quixotic political positions and the expectation this year's campaign will be decided by thin margins, both Democrats and Republicans worry that he could play the role of spoiler.

Biden planned to use Thursday's event, which caps a three-day campaign swing in a battleground state critical to his reelection effort, to also sustain the pressure on Trump, the former Republican president.

"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," according to Kerry Kennedy's prepared remarks. "Daddy stood for equal justice, human rights and freedom from want and fear. Just as President Biden does today."

The endorsement was hardly a surprise. Members of the prominent Democratic family have been vocal that they don't see eye to eye politically with Kennedy Jr., who started as a protest primary challenger to Biden in the Democratic Party and now is running as an independent. Biden last month hosted more than 30 members of Kennedy's extended family at the White House for St. Patrick's Day, when family members posed with the president in the Rose Garden and Oval Office.

After the formal endorsement, Biden and members of the Kennedy family were to meet with supporters at a campaign event, and members of the Kennedy clan were planning to make calls to voters and knock on doors on Biden's behalf.

Several notable members of the family were not endorsing, including Caroline Kennedy, the U.S. ambassador to Australia, and nonprofit leader Maria Shriver, which the Biden campaign said was due to their nonpolitical professional roles.

Shriver, however, has been a conspicuous White House guest recently, attending the State of the Union and speaking at a women's history month reception last month.

Bernard Tamas of Valdosta State University, an expert on third parties, said it was unclear whether Kennedy Jr. would pull more votes from Democrats or Republicans.

"He is pro-science when it comes to the environment, but a conspiracy theorist when it comes to vaccines," Tamas said.

Kennedy Jr.'s lack of a clear political lane limits his potential impact on the election, Tamas said, but Democrats appear to be more concerned because his last name could lead some voters to believe that he is carrying on his family's political legacy.

Other than that, Tamas said, "I don't know what else he has to attract progressive voters."

Kennedy Jr. has spoken publicly in the past about disagreeing with his family on many issues, but maintains it can be done in "friendly" ways. After a super political action committee supporting his campaign produced a TV ad during the Super Bowl that relied heavily on imagery from John F. Kennedy's 1960 presidential run, Kennedy Jr. apologized to his relatives on the X social media platform, saying he was sorry if the spot "caused anyone in my family pain."

The Democratic National Committee has hired a communications team to combat the appeal of third-party candidates, Kennedy Jr. first among them. The DNC also filed a recent Federal Election Commission complaint against Kennedy Jr.'s campaign, charging that it coordinated too closely with an affiliated super PAC to get his name on the presidential ballot in some states.

Kennedy Jr. is also viewed warily by the Trump campaign. While Trump has released a recent video saying, "If I were a Democrat, I'd vote for RFK Jr. every single time over Biden," he has sometimes criticized Kennedy Jr. as being more "radical left" than Biden.

The Kennedy family endorsement is a capstone on three days of campaigning in Pennsylvania.

Biden's travels were an opportunity to reconnect with his roots, starting on Tuesday in Scranton, where he lived until he was 10 years old. He swung by his childhood home, a three-story colonial that his family rented, and reminisced about attending Mass at St. Paul's.

He seemed reluctant to leave town the next day, stopping for coffee before heading to the airport. "It's good to be back in Scranton," the president said when a customer welcomed him.

Biden's next stop was Pittsburgh, where he called for higher tariffs on steel and aluminum from China to protect U.S. industry from what he called unfair competition.

But even that event involved some nostalgia, as Biden recalled an endorsement from the steelworkers when he was "a 29-year-old kid" from Delaware running for U.S. Senate.

"It changed everything," he said.