Members of the Kennedy family appeared en masse and in force to endorse President Biden at a campaign event in Philadelphia on Thursday.

Why it matters: It's a public display of support that reveals a weakness Democrats privately acknowledge: Biden is likely more at risk from Robert F. Kennedy Jr.'s third-party challenge than former President Trump is.

• With a substantial number of voters telling pollsters they are dissatisfied with both Biden and Trump, Democrats are increasingly petrified that a Kennedy name on the ballot will throw the election to Trump.
• A similar fear pervaded over the potential No Labels unity ticket. But with those plans scuttled, Biden is turning his focus on the scion of the storied Kennedy family.

Driving the news: RFK Jr.'s sister, Kerry Kennedy, invoked the legacy of her slain father, former Attorney General Robert F. Kennedy, as well as that of her uncles: former President John F. Kennedy and Sen. Ted Kennedy (D-Mass.).

• "I can only imagine how Donald Trump's outrageous lies and behavior would have horrified my father," she said.
• "President Biden has been a champion for all the rights and freedoms that my father and uncles stood for," she added.

The big picture: Kerry Kennedy lauded Biden's track record fighting for "working people," his support for unions, and his efforts to relieve "the debt incurred by middle-and-working class kids trying to get ahead by getting an education."

• By contrast, if Trump wins he will "incite more chaos, division, and political violence with his extreme agenda," she said.
• In a veiled dig at her brother's campaign, Kennedy added that "in 2024 there are only two candidates with any chance of winning the presidency."

State of play: "What an incredible honor, to have the support of the Kennedy family," Biden said when he took the stage.

• After Biden's remarks, Kennedy family members plan to knock on doors and make calls on behalf of the Biden campaign.

Zoom out: From inflation to abortion, the Biden campaign is seizing every opportunity to make 2024 a choice between two candidates — and two competing visions for America.

• Kennedy's independent bid threatens that strategy.
• Democrats view anything that turns the election into a referendum on Biden — and gives voters a chance to register their discontent with the status quo — as a grave threat to Biden's candidacy.
• The event on Thursday is part of an increasingly aggressive effort to take on Kennedy, with most polls showing Trump's advantage grows with the ex-Democrat's famous last name on the ballot.
• The Democratic Party is also preparing to fight Kennedy's attempts to get on the ballot in key swing states, and has hired communications professionals at the DNC with the explicit aim of combatting Kennedy.

Zoom in: Biden advisers aren't concerned about Kennedy making deep inroads with Democrats and independents. They're worried about his effect on the margins in a few key swing states.

• Two of the last three GOP presidential victories — President Bush in 2000 and Trump in 2016 — can partly be explained by the appeal of third-party candidates to disaffected voters.
• The electoral math bolsters those fears.
• Trump's 2016 margin of victory over Hillary Clinton in Michigan, Pennsylvania and Wisconsin was smaller than the total number of votes won by Green Party candidate Jill Stein in those states.

What we're watching: Kennedy is actually sounding a lot more like Trump — with pledges to "seal the border" and appoint a special counsel to review Jan. 6 prosecutions — than a traditional Democrat from a storied family.

• But Republicans are also concerned about Kennedy's potential crossover appeal, with pro-Trump media abruptly shifting its tone on his candidacy.

The bottom line: Many Kennedy family members have been critical of Robert F. Kennedy Jr.'s candidacy.

• Biden posed with dozens of them, including Kerry Kennedy, at a St. Patrick's Day party at the White House last month.
• Thursday's event will consolidate that criticism and try to turn it into positive momentum for Biden.

Editor's note: This story has been corrected to say that President Bush was elected in 2000, not 2020. It has also been updated with the latest news.