Could John Fetterman Win a Defamation Lawsuit Against Donald Trump, for Accusing Fetterman of Hard Drug Use?

At a campaign rally, Trump said,

Fetterman supports taxpayer-funded drug dens and the complete decriminalization of illegal drugs, including heroin, cocaine, crystal meth, and ultra lethal fentanyl. By the way, he takes them himself.

The clip included above seems to support that. If the "he takes them himself" statement is false, could Fetterman (a public official) win a defamation lawsuit against Trump?

Yes, though he'd have to show, by "clear and convincing evidence," that Trump spoke "with knowledge that it was false or with reckless disregard of whether it was false or not." "Reckless disregard" in turn refers to a "high degree of awareness of … probable falsity" or "entertain[ing] serious doubts as to the truth of his publication."

"[F]ailure to investigate before publishing, even when a reasonably prudent person would have done so, is not sufficient to establish reckless disregard." But "[a]lthough failure to investigate will not alone support a finding of actual malice, the purposeful avoidance of the truth is in a different category." " [A] deliberate decision not to acquire knowledge of facts that might confirm the probable falsity of [the] charges" may well qualify as reckless disregard. (These quotes are from Harte-Hanks v. Connaughton (1989), a convenient summary by the Court of the misleadingly named "actual malice" test, which was set forth by New York Times v. Sullivan (1964).)

Now this is a subjective test—what did the speaker actually believe, and deliberately decide?—and not an inquiry into what a reasonable speaker would have done. Still, under the right circumstances, a jury can infer that the speaker must have realized the accusation was probably false (rather than just that he should have realized it), or must have deliberately decided not to investigate, and the jury can disbelieve a speaker's claim that he was sincerely sure the statement was false.

So, if Fetterman can persuade the jury the accusation was false, and can also persuade the jury (again, by clear and convincing evidence) that Trump knew it was false or probably false, Fetterman would win. (I say "would" on the assumption that the jury follows the instructions.)

On the other hand, if Fetterman can't do so—perhaps because Trump can point to some source for the accusation that the jury thinks he actually believed (whether or not he should have believed it)—then Fetterman would lose. And the case likely would go to the jury, if it's a question of whose claims or denials to believe.

Some have pointed out that this rewards the crazy or the foolish, who actually sincerely believe unreasonable claims. But that is the nature, for better or worse, of the New York Times v. Sullivan subjective test.

Note that sometimes such allegations are clearly facetious or jocular; if a reasonable listener would indeed understand them that way, then they aren't actionable. The same is true for parodies, see New Times v. Isaacks (Tex. 2004), the "Where the Wild Things Are" case. But I don't see any evidence of that here.

Of course, none of this tell us whether it's a wise move for Fetterman to sue, whether before the election or after. But that's the general legal framework.