The last time an American was convicted of treason was in the 1940s, in part because the Constitution defines it narrowly as levying war against the United States or giving aid or comfort to an enemy during wartime. But as the impeachment inquiry unfolds, Mr. Trump has used the term to accuse people of disloyalty and signal to his supporters that his political opponents are un-American.

The accusation is nothing new in the right-wing vocabulary — Ann Coulter, the president’s sometimes adviser, sometimes adversary — wrote a book on the topic. Mark S. Zaid, a Washington lawyer who represents the whistle-blower, reviewed the book in 2003, saying it contained revelations that would “shock anyone to the left of Attila the Hun.”

But experts see a civic danger in the president frequently and falsely calling out what he labels treasonous behavior. Carlton F.W. Larson, a professor at the University of California at Davis School of Law and an expert on treason, said the president had cheapened the word through overuse.

“It is a complete debasement of political discourse where ordinary political disagreements are elevated to a level of capital crime,” Mr. Larson said. “If Trump ever did uncover an instance of true treason, no one would believe him because he has so debased the meaning of that word.”

Mr. Trump first mentioned treason in connection with the whistle-blower last week in a talk at the United States Mission to the United Nations, contending that what the whistle-blower was doing was spying and lamenting that there used to be a punishment for that, presumably meaning execution.