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.”