The prospect of Robert F. Kennedy Jr. threatening to upend the presidential race went from an idea to a reality in one of the country’s most consequential battlegrounds on Thursday, when Mr. Kennedy qualified for the ballot in Michigan.
The decision by the Natural Law Party to grant Mr. Kennedy its ballot line in November ensures he will be a factor in a pivotal swing state where the presidential election is expected to be incredibly close and where President Biden has already shown vulnerability with key Democratic constituencies. During Michigan’s primary in February, a protest movement over Mr. Biden’s support for Israel’s war in Gaza drew significant support.
Mr. Kennedy, a lifelong Democrat and the scion of perhaps the nation’s most famous Democratic family, is running as an independent in 2024 and polling higher in early surveys than any third-party candidate since Ross Perot, the self-funding billionaire who ran in the 1990s.
His independent candidacy has earned him the estrangement of his own family — who campaigned this week with Mr. Biden in Pennsylvania — and many of his previous colleagues from the environmental movement, who denounced his candidacy publicly on Friday.
Mr. Kennedy has emerged as a leading voice of vaccine skepticism, both before and after Covid-19 vaccine, and dabbled in conspiratorial theories, including about the assassination of Robert F. Kennedy, his father. He had initially declared his 2024 campaign as a Democratic challenger to Mr. Biden — and was heavily featured in conservative media — until he switched last fall to run as an independent.
Both parties are already rushing to define Mr. Kennedy as he races to get on the ballot nationwide.
Brian Hughes, an adviser to Donald J. Trump, the presumptive Republican nominee, called Mr. Kennedy “a leftist and liberal with a history of supporting an extreme environmental agenda that rivals Joe Biden for its ability to kill American jobs.”
Lis Smith, a Democratic strategist working with the Democratic National Committee, said the Democrats were working to ensure voters “know about how extreme his stances are and that he was recruited to run by Trump allies and funded by Trump’s largest donor to be a spoiler for Trump in November.” Timothy Mellon, the reclusive banking heir, has given $15 million to a pro-Trump super PAC and $20 million to a pro-Kennedy super PAC.
The Biden operation is taking the threat of Mr. Kennedy seriously, inviting a phalanx of the Kennedy clan to the White House on St. Patrick’s Day and then on Thursday holding an event in Philadelphia, where Mr. Kennedy’s relatives, including six of his siblings, threw their support behind the president.
“We cannot do anything that in any way strips even one vote from President Biden,” Joseph P. Kennedy II, Mr. Kennedy’s eldest brother and a former Democratic congressman from Massachusetts, said in an interview. “You put the name Kennedy on the ballot, and Democrats are going to feel torn.”
On Sunday, Mr. Kennedy will celebrate gaining access to the Michigan ballot by hosting a comedy show outside Detroit, his campaign said in a fund-raising message. The event will be headlined by Rob Schneider, the Hollywood comedian who has questioned the safety of vaccines. Mr. Kennedy’s campaign did not respond to a request for comment.
Some Michigan Democrats expressed concern about Mr. Kennedy’s presence on the ballot, given the tight margins, his famous name and Mr. Biden’s struggles to reunite the coalition that carried him to victory in 2020.
“In a very close race, we could be talking about a few thousand votes that could swing the results one way or another,” said Representative Shri Thanedar, who represents a Detroit-area district. “There is already some discontent among the Muslim Americans. There’s some discontent among African American voters. So this is very critical.”
“The more voters learn about R.F.K. Jr, the less likely they will be to support him,” Ms. Smith said.
Mr. Hughes noted that it was Democrats who were “using financial and legal resources to prevent candidates’ access to the ballot.”
“We are confident we can win support of the voters no matter who is on the ballot,” he said.
Polling is mixed on how Mr. Kennedy may change the race.
A series of Fox News polls in swing states showed that his inclusion — along with that of other third-party candidates — sometimes helped Mr. Trump, sometimes Mr. Biden and sometimes neither.
The Fox News survey in Michigan showed Mr. Kennedy at 9 percent, Mr. Trump in the lead at 42 percent, and Mr. Biden at 40 percent. Two other third-party candidates, the independent Cornel West and Jill Stein of the Green Party, were at 2 percent. In a head-to-head race with Mr. Biden in the state, Mr. Trump led 49 percent to 46 percent.
Even how to track Mr. Kennedy’s popularity has been the subject of fierce debate as pollsters try to find ways to neither inflate nor undercount his support, with different methods leading to different results.
Michigan is the first battleground where Mr. Kennedy has made the ballot, and the state is expected to see tens of millions of dollars spent in 2024 after razor-thin recent contests. Mr. Trump won the state by only 10,704 votes in 2016. Mr. Biden put it back in the Democratic column by around 154,000 votes in 2020.
It is also the state that has exposed some of the deepest divisions within the Democratic Party over Israel, Hamas, the invasion of Gaza and Mr. Biden’s handling of the conflict.
In Michigan’s Democratic primary, a protest movement urging voters to cast their ballots for “uncommitted” received 13 percent of the vote, enough to earn two delegates to the party’s national convention. The movement gathered support from younger voters, progressives and the state’s influential Arab American and Muslim population, which is especially sizable in Dearborn.
In interviews during the primary, many of those voters said they planned to sit out the election or vote for a third-party candidate rather than for Mr. Trump or Mr. Biden.
But Mr. Kennedy may not be the ideal candidate to take advantage of those disaffected Democrats.
He has taken a more hawkish pro-Israel position that, if widely publicized, could make it difficult for him to harness anti-Biden sentiment in Arab American communities. In one interview in late 2023, Mr. Kennedy said that “the Palestinian people are arguably the most pampered people by international aid organizations,” and more recently he described Israel as “a moral nation.”
“R.F.K. is someone who is, quite frankly, not welcome in Michigan,” said Abbas Alawieh, a former Democratic staffer on Capitol Hill who helped lead the state’s Listen to Michigan protest effort. “His campaign is a vanity project. He is not a friend of our movement.”
In a recent Wall Street Journal poll of battleground states, Mr. Trump led by three percentage points in a head-to-head race with Mr. Biden in Michigan and by two points when Mr. Kennedy was included in the survey.
“Third parties can always go both ways, and R.F.K. brings a little bit for a lot of voters,” said Stu Sandler, a Republican strategist who has done extensive work in Michigan. “At the moment, it certainly seems this will help Trump. There is a lot of disaffected Biden voters, particularly if you look at areas like Dearborn, over Biden’s support of Israel.”
Representative Haley Stevens, a Michigan Democrat, said the Biden campaign should remain focused on Mr. Trump, suggesting that efforts to attack Mr. Kennedy could elevate his campaign.
But she also said Democrats could not “totally ignore” him either.
“I am curious to see if he’ll set up an operation in Michigan and if he will have fund-raised enough to be able to sell a campaign in Michigan,” Ms. Stevens said. “And frankly, it’s also unclear who he’s really going to take from.”
For now, the Kennedy campaign is focused on getting on the ballot in more states.
Last month, he named as his running mate Nicole Shanahan, a lawyer and the wealthy ex-wife of one of Google’s co-founders. She quickly plunged $2 million into the campaign, which is expected to help Mr. Kennedy with the costly and arduous task of getting on the ballot nationwide.