He noted that opponents labeled themselves “the resistance” immediately after Mr. Trump was elected and accused them of “using every tool and maneuver to sabotage the functioning of the executive branch and his administration.

“Resistance is the language used to describe insurgency against rule imposed by an occupying military power,” Mr. Barr said. He added that it connotes that the government is not legitimate. “This is a very dangerous and indeed incendiary notion.”

Mr. Barr spoke as the second public impeachment hearing wrapped up on Capitol Hill, where Democrats have accused Mr. Trump of abusing the power of his office for personal gain.

Marie L. Yovanovitch, the former ambassador to Ukraine, testified that she was the target of a smear campaign engineered to get Mr. Trump to remove her; she was recalled from Kiev in the spring. She said that her dismissal from the post put national security at risk by opening the door for Russia to further influence Ukraine, a strategic American ally.

She also said that she felt devastated and threatened to learn that Mr. Trump had vilified her to Mr. Zelensky, testimony that Mr. Trump underscored by attacking her on Twitter as she sat before lawmakers.

In his address, Mr. Barr suggested the president has acted within his powers and that his opponents were willing to bend the law to stop him.

Mr. Barr is known as an executive power maximalist and a believer in the unitary executive theory, which posits that the Constitution imbues the presidency with broad powers that are subject to relatively little oversight.