Let’s start with the easiest question. Was Trump “inciting” violence against Clinton? In any ordinary-language of the term, the answer is probably yes. He was signaling to the unhinged fringe—a group he has courted with birtherism, Clinton conspiracy theories, libels against American Muslims, and a lot of coy eyelash-batting in the direction of David Duke and other white supremacists. The signal was (to be as generous as possible), “I share your anger and also wonder whether it will be someday be necessary for you to kill your enemies.”
Does that contribute to the possibility of violence? Yes. Does it intentionally encourage those who believe in killing for political aims? Yes.
Does it violate laws against “incitement”? No.
The reason is that, under American law, “incitement” is a narrow term. Until the 1960s, most states in the U.S. had “criminal syndicalism” laws that made it a crime to “advocate”—or even, in some states, merely associate with groups that advocated—political or economic change by violence or revolution. The law was handy for crushing labor groups like the Industrial Workers of the World; the U.S. government used its federal analogue, the Smith Act, to destroy the leadership of the Communist Party.
In 1969, the Supreme Court voided these laws in a case called Brandenburg v. Ohio. The defendant had been convicted of making a speech to a KKK “rally” (attended by 12 Klansmen and a TV crew on a private farm) in which he announced, “We're not a revengent organization, but if our president, our Congress, our Supreme Court, continues to suppress the white, Caucasian race, it's possible that there might have to be some revengeance taken.” The Court held that “mere advocacy”—even of violence—cannot be made criminal “except where such advocacy is directed to inciting or producing imminent lawless action and is likely to incite or produce such action.”
Whatever Trump’s intent in Wilmington, the “imminence” requirement is the bar to incitement charges against Trump. What the Court meant is that the First Amendment requires a showing that the violence was literally about to occur at that very moment. “Kill that person over there right now!” is the template.
By that standard, did Trump “incite” violence against Clinton? No.
What about “fighting words”? That’s a term that gets thrown about but that constitutionally is virtually nonexistent. It means, in essence, speech that proposes an immediate fistfight and is thus not protected by the First Amendment. (A stranger once came up to me on the street and said, “I’m going to knock the crap out of you.” Those are “fighting words”; what Trump said was not.)
More difficult to assess is whether the words are a threat. Federal law (18 U.S.C.A. § 875) makes it an offense to transmit “in interstate or foreign commerce any communication containing … any threat to injure the person of another”; beyond that, a specific statute, 18 U.S.C. § 879, says that anyone who “knowingly and willfully threatens to kill, kidnap, or inflict bodily harm upon ... a major candidate for the office of President” is guilty of a felony, punishable by up to five years in prison.