WASHINGTON — When Donald J. Trump was running for president in 2016, he pointed to some protesters at one of his rallies and told the crowd to “get ’em out of here.” The protesters, who said they were then viciously assaulted, sued him for inciting a riot.

Mr. Trump won the suit. A federal appeals court, relying on a case concerning the Ku Klux Klan, ruled that his exhortation was protected by the First Amendment. And now his lawyers are making the same argument at his impeachment trial, where he stands accused of inciting an insurrection.

But Democrats say that argument misses two key points. An impeachment trial, they contend, is concerned with abuses of official power, meaning that statements that may be legally defensible when uttered by a private individual can nonetheless be grounds for impeachment.

Equally important, they say that Mr. Trump’s statements on Jan. 6 should not be considered in isolation but as the final effort of a calculated, monthslong campaign to violate his oath of office in an effort to retain power.

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Precisely because the definition of incitement is so vague, the Supreme Court has placed strict constitutional limits on lawsuits and prosecutions seeking to punish it.

In 1969, in Brandenburg v. Ohio, for instance, the court unanimously overturned the conviction of a leader of a Ku Klux Klan group under an Ohio statute that banned the advocacy of terrorism. The Klan leader, Clarence Brandenburg, had urged his followers at a rally to “send the Jews back to Israel,” to “bury” African-Americans, though he did not call them that, and to consider “revengeance” against politicians and judges who were unsympathetic to white people.

Only Klan members and journalists were present. Because Mr. Brandenburg’s words fell short of calling for immediate violence in a setting where such violence was likely, the Supreme Court ruled that he could not be prosecuted for incitement.

“The constitutional guarantees of free speech and free press,” the court said in an unsigned opinion, “do not permit a state to forbid or proscribe advocacy of the use of force or of law violation except where such advocacy is directed to inciting or producing imminent lawless action and is likely to incite or produce such action.”

Judge Hale said the account of the rally presented in the protesters’ lawsuit could clear the high bar. “It is plausible that Trump’s direction to ‘get ’em out of here’ advocated the use of force,” the judge wrote. “It was an order, an instruction, a command.”