PHILADELPHIA — 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 candidate, delivered the endorsements in Philadelphia by calling Biden “my hero.”

“We want to make crystal clear our feelings that the best way forward for America is to reelect Joe Biden and Kamala Harris for four more years,” she said.

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 honor.” He said Trump, the former president who is the presumptive Republican 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’s support more than six months from Election Day is an indication of how seriously Biden’s team is taking 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 be a spoiler.