STORY: "A vote for Joe Biden is a vote for our democracy and our decency."

In a rebuke of Robert F. Kennedy Jr's independent bid for the White House, 15 members of the Kennedy family endorsed U.S. President Joe Biden at a campaign event in Philadelphia on Thursday.

"I can only imagine how Donald Trump's outrageous lies and behavior would have horrified my father."

Here's RFK Jr's younger sister Kerry Kennedy:

"We want to make crystal clear our feeling that the best way forward for America is to reelect Joe Biden and Kamala Harris to four more years. [FLASH] Donald Trump is running to take us backwards, attacking the most basic rights and freedoms that are at the core of who we are as Americans."

Many in RFK Jr's storied political family - prominent Democrats since the 19th century - have broken with him over his views.

The former environmental lawyer turned anti-vaccine advocate acknowledged on social media that he had heard some of his family was endorsing Biden, writing:

"I am pleased they are politically active - it's a family tradition... We are divided in our opinions but united in our love for each other."

According to a recent Reuters/Ipsos poll, RFK Jr - son of the slain U.S. senator and presidential candidate Robert F. Kennedy - is backed by 15% of registered voters, versus 39% for Biden and 38% for Trump.

At Thursday's event with the Kennedys, Biden did not directly mention RFK Jr's bid for the White House, describing the presidential election as a two-person race.

Biden: "The 2024 election is about two fundamentally different visions for America. Donald Trump's vision is one of anger, hate, revenge and retribution. [FLASH] I see an America where we defend Democracy, not diminish it. I see an America where we protect our freedoms, not take them away."

Thursday's campaign event as well as efforts by the Make America Great Again Inc super PAC to paint RFK Jr as a "leftist radical" underscore concerns both parties have that Kennedy could take votes away from their candidate.

The Philadelphia event was part of Biden's third day this week in Pennsylvania, a critical swing state for his re-election bid.

Biden needs strong turnout from Philadelphia's Black community to win the state, and the Kennedys became icons because of their advocacy for civil rights.