Robert F. Kennedy Jr. will be on the general election ballot in Michigan.

On the same day that more than a dozen members of his family endorsed President Biden, Robert F. Kennedy Jr. announced Thursday that he had secured a spot on the ballot in Michigan, one of the swing states likely to decide the election.

The Natural Law Party, which has ballot access in Michigan, nominated Mr. Kennedy and his running mate, Nicole Shanahan, for president and vice president. A spokeswoman for the Michigan secretary of state’s office confirmed that this meant Mr. Kennedy would be on the ballot in November.

Mr. Kennedy is seeking ballot access in all 50 states through a mixture of independent petitions and third-party nominations. In some states, he has filed to create his own party. At the same time, Democratic allies of Mr. Biden are working to challenge his access to the ballot.

In a statement released by Mr. Kennedy’s campaign, Doug Dern, the chairman of the Natural Law Party, called him “the most qualified candidate in the modern-day history of America.”

Mr. Kennedy, an environmental lawyer known for his antivaccine activism, has spread misinformation about the risks of vaccinations and about other subjects. He also drew attention recently for suggesting that Mr. Biden might pose a greater threat to democracy than former President Donald J. Trump, who tried to overturn the results of the 2020 election, a claim that democracy experts called absurd.

While Mr. Kennedy is generally polling in single digits nationally, Mr. Biden’s campaign is worried that he could draw enough support in battleground states like Michigan to tip a close election to Mr. Trump.

Mr. Kennedy’s relatives are worried about that, too — as evidenced by their show of force for Mr. Biden on Thursday, when a group of them appeared alongside him at a campaign event in Pennsylvania.