U.S. President Joe Biden scooped up endorsements from at least 15 members of the Kennedy political family during a campaign stop Thursday 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 contender, delivered the endorsements in Philadelphia by calling Biden "my hero."

"We want to make crystal-clear our feeling that the best way forward for America is to re-elect Joe Biden and Kamala Harris," Kerry Kennedy said Thursday.

She never directly mentioned her brother, but insisted "there are only two candidates with any chance of winning the presidency" this year, framing the campaign as a choice between Biden and Trump, with no room for a third-party contender.

Biden, who keeps a bust of Robert F. Kennedy in the Oval Office, said the endorsements were "an incredible honour." He said Trump, the former Republican president who is the presumptive GOP nominee, was a threat to America's democratic traditions and that "now it's time to keep going and not slow down because there's so much at stake."

The decision to highlight the Kennedy family endorsement more than six months from election day is an indication of how seriously Biden's team is taking the threat of the long shot bid potentially using his last name's lingering Democratic magic to siphon off support from the incumbent president.

Kennedy Jr. downplays endorsements

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. As a reminder of that potentially decisive role, officials in Michigan said Kennedy qualified for the state ballot on Thursday.

Biden used Thursday's event, which capped a three-day swing in a battleground state critical to his re-election effort, to keep up the pressure on Trump.

"Donald Trump's vision is one of anger, hate, revenge and retribution," Biden said, adding, "I have a very different view of America, one of hope and optimism."

After the event, Biden thanked about three dozen supporters and volunteers who were gearing up to call voters or knock on doors for his campaign.

"What you're doing here is bigger than me, bigger than you, bigger than all of us combined. It's about what kind of country our kids are going to live in," Biden said.

He portrayed the election as less about keeping himself in office than keeping Trump out, saying the race was important "not because I'm running" but because of "what happens if we lose this election."

The Kennedy family endorsements are 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, with family members posing with the president in the Rose Garden and Oval Office.

Later, Biden and members of the Kennedy family were to meet with supporters at a campaign event, and some 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 U.S.ambassador to Australia Caroline Kennedy and non-profit leader Maria Shriver, which the Biden campaign said was due to their non-political 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 PAC supporting his campaign produced a TV ad during the Super Bowl that relied heavily on imagery from John F. Kennedy's 1960 presidential run, Robert Kennedy Jr. apologized to his relatives on the social media platform X, saying he was sorry if the spot "caused anyone in my family pain."

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

Kennedy 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 also sometimes criticized Kennedy as being more "radical left" than Biden.

On his way out of Philadelphia, Biden swung by a Wawa convenience store, accompanied by Mayor Cherelle Parker, where he picked up some food and ordered a black and white milkshake. That served to cover Biden's bases on one front, given his stop Wednesday in Pittsburgh at a Sheetz, a rival convenience chain.

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

In Pittsburgh, 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.