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.

His supporters have created a new party, We the People, to help him secure ballot access in a few states: California, Delaware, Hawaii, Mississippi and North Carolina.

Does R.F.K. Jr. pose a bigger threat to Biden or to Trump?

That this is such a common question — and that both sides are concerned about it — reflects what a political enigma Mr. Kennedy has become and the range of people who have been drawn to his candidacy. Still, it is hard to discern whether he would draw more voters in a general election from Mr. Trump or from Mr. Biden.

How worried is the Biden campaign about R.F.K. Jr.? The Trump campaign?

Democrats have been haunted by third-party candidates since 2000, when Ralph Nader, running with the Green Party, was partly blamed for costing Al Gore the election. In 2016, Jill Stein — also with the Green Party — won more than 30,000 votes in Wisconsin, more than Mr. Trump’s margin of victory over Hillary Clinton in that state.

Fears among Democrats are particularly acute this year, with polls suggesting that Mr. Trump’s base of enthusiastic support is sturdier than Mr. Biden’s. Conventional wisdom within the party is that any vote not for Mr. Biden benefits Mr. Trump, and there are concerns that giving people more choices on the ballot — especially one with the last name Kennedy — is more likely to hurt Mr. Biden, especially in critical swing states. The party is taking aggressive measures to counter his candidacy.

Where is Kennedy on the ballot?

As of early April, Mr. Kennedy is officially on the ballot in one state: Utah. His campaign says he has enough signatures to get on the ballot in Hawaii, Idaho, Nevada, New Hampshire and North Carolina, but he still has to submit those signatures in applications to state officials.

His campaign is gathering signatures in most other states. The time frame to do so in some places, like Colorado and Louisiana, is narrower and has not started yet.

Each state has different rules for ballot access, and they vary depending on whether a candidate is an independent or with a third party. Almost all of the rules revolve, ultimately, around signatures: Candidates and parties have a certain window of time to gather tens of thousands of signatures, which must be submitted for approval by state authorities. And signatures take time, labor and money — tens of millions of dollars, by most estimates, including the cost of litigating challenges to ballot applications.

Who is his running mate?

Mr. Kennedy has named Nicole Shanahan, a Bay Area lawyer and investor, as his running mate. Ms. Shanahan, who was once married to the Google co-founder Sergey Brin, is a political newcomer, having never sought or held public office.

Ms. Shanahan, 38, does have a history of political giving — she gave to Mr. Biden’s 2020 campaign. And she has already shown her willingness to provide financial support to Mr. Kennedy’s campaign: She has given more than $4.5 million to super PACs backing him, including the bulk of the funding for a Super Bowl ad that one of the PACs bought.

Ms. Shanahan has aligned herself with issues and positions that are important to Mr. Kennedy, and which have animated his campaign and his core followers, including skepticism of vaccines, concerns about chronic disease, disaffection with the Democratic Party and environmental stewardship.

Who is R.F.K. Jr. popular with?

Mr. Kennedy’s supporters fall into a few overlapping categories. First, there are people who have supported his work with Children’s Health Defense — an activist group primarily known for spreading anti-vaccine disinformation.

More broadly, he has ardent, longtime supporters in the so-called medical freedom movement, which draws a cross section of political beliefs (including those of Libertarians and people who are distrustful of mainstream medicine) through its opposition to vaccination requirements and public health measures.

More recently, his outspoken criticism of the government’s handling of the pandemic — including skepticism about the Covid vaccines, broadsides against top public health officials like Dr. Anthony S. Fauci, and his protests of lockdowns and monitoring of disinformation — have drawn new supporters from across the political spectrum.

Some of his supporters are longtime Democrats drawn to the Kennedy luster — Mr. Kennedy has anchored his campaign in a mythmaking nostalgia for his family, even though most members of his family have publicly disavowed his candidacy.

Mr. Kennedy has also drawn support from Libertarians and independents who are broadly distrustful of the federal government — he expresses views that align with those of many Trump supporters, including an isolationist foreign policy and outrage over purported censorship in the news media and on tech platforms. He has become known for confrontational, provocative interviews on right-leaning podcasts, and has a kind of iconoclastic anti-establishment vibe that has appealed to disaffected Democrats and Republicans.