Donald J. Trump’s decision to give Robert F. Kennedy Jr. a cabinet post appeared to fulfill his promise to Mr. Kennedy to find him a spot in his administration.

Mr. Trump may well have overpaid in the deal.

Pre-election polling was inconclusive about whether Mr. Kennedy’s exit from the race gave a more significant advantage to Mr. Trump or Vice President Kamala Harris. A post-election analysis of results offered few clues to answer that question.

And though the electoral value of Mr. Kennedy’s endorsement may be inconclusive, it always had more of a symbolic than a real-world appeal to Mr. Trump. Winning the backing of Mr. Kennedy — an anti-vaccine conspiracy theorist and the scion of Democratic royalty — supplied Mr. Trump not necessarily with votes in key swing states but with a fuzzier owning-the-libs morale boost and the kind of name-brand approval he often craves.

Mr. Trump selected Mr. Kennedy as the next secretary of health and human services, the type of cabinet post typically reserved for political allies who deliver a battleground state or a crucial coalition of voters. Mr. Kennedy has said that “there is no vaccine that is, you know, safe and effective,” a view at odds with most of the country regarding childhood vaccinations. He ran a 16-month campaign for president and dropped out in August as his troubled independent bid was running out of money.

In his first administration, Mr. Trump basked in his ability to stock his cabinet with “killers,” as he referred to them — corporate executives, Wall Street financiers and four-star generals, all of whom reported to Mr. Trump and, to remain in his good graces, often lavished him with praise.

As Mr. Trump builds out a second administration, Mr. Kennedy is the clearest example of that history repeating itself.