Mr. Kennedy was formally nominated at a brief convention held Wednesday morning in Mr. Dern’s law office. The only two attendees were Mr. Dern and the party’s secretary.

Stefanie Spear, a spokeswoman for the Kennedy campaign, confirmed the timeline of the campaign’s outreach and said simply that Mr. Kennedy and his running mate, Nicole Shanahan, had been “duly nominated by the Natural Law Party.”

Third parties have drawn increasing attention in recent elections — and arguably have swung some of them — by allowing voters to express discontent with major-party candidates. Mr. Kennedy is counting on a particularly strong showing in this election as polls show voters are highly dissatisfied with their options.

The Natural Law Party was founded in 1992 on a platform that included promotion of transcendental meditation, responsible gun use, flat taxes and organic farming. But it has dwindled over the years. Heading into the 2024 election, Michigan’s was its only state chapter.