What to Know About R.F.K. Jr. and His Threat to Biden and Trump

The independent presidential candidate Robert F. Kennedy Jr. has emerged as a wild card of the 2024 election, attracting a motley mix of ideologically diverse supporters, raising piles of cash and drawing legal attacks from Democrats and verbal barrages from former President Donald J. Trump.

Mr. Kennedy, 70, the son of Robert F. Kennedy and an heir to an American political dynasty, had a troubled youth and young adulthood marked by drug abuse. He became an environmental lawyer, most famous for suing corporate polluters in an effort to clean up the Hudson Valley watershed.

In the past decade, he has become a prominent voice in the anti-vaccine movement, promoting falsehoods and conspiracy theories about the risks of childhood vaccinations and other public health measures. That work gave him a large platform during the coronavirus pandemic, when he questioned the safety of Covid vaccines and the official narratives of the virus’s origins.

With the centrist group No Labels announcing on April 4 that it would not run a presidential ticket, Mr. Kennedy is the most prominent independent or third-party presence in the 2024 race. Here’s what to know about him, his supporters and how President Biden’s and Mr. Trump’s campaigns are approaching him.

What party is R.F.K. Jr. affiliated with?

Mr. Kennedy is running as an independent, so he is not affiliated with an established political party — he is not even, technically speaking, a “third-party candidate.” In keeping with his family’s political legacy, Mr. Kennedy was a lifelong Democrat, and when he entered the race in April 2023, he sought to challenge Mr. Biden for the party’s nomination. Six months later, he announced that he would run as an independent, saying the Democrats had corruptly blocked his efforts.

He has flirted with the Libertarian Party, which is on the ballot in about three dozen states. If he were to join its ticket, his efforts to get on states’ ballots would become much simpler.