Attorney General William Barr on Friday offered a strident defense of President Trump's use of executive authority and slammed House Democrats' impeachment investigation as an effort to undermine him.

"While the President has certainly thrown out the traditional Beltway playbook, he was upfront about that beforehand, and the people voted for him," Barr said in his prepared remarks at a conference hosted by the conservative Federalist Society.

"The fact is that this Administration's policy initiatives and proposed rules, including the Travel Ban, have transgressed neither constitutional, nor traditional, norms, and have been amply supported by the law and patiently litigated through the Court system to vindication," he added.

While Barr never specifically mentioned impeachment, he took the president's critics to task for what he said was an effort to undermine Trump.

"Immediately after President Trump won election, opponents inaugurated what they called 'The Resistance,' and they rallied around an explicit strategy of using every tool and maneuver available to sabotage the functioning of his Administration," he said. "They essentially see themselves as engaged in a war to cripple, by any means necessary, a duly elected government."

Barr is a well-known subscriber to the unitary executive theory, which postulates that the president is granted broad powers under the Constitution and that there are relatively few mechanisms to pursue oversight of the White House.

The attorney general has argued in the past that Congress cannot make a law against the president exercising executive powers for corrupt purposes and that presidents have authority over investigations even when those probes are focused on their activity.

"This is not 'new,' and it is not a 'theory.' It is a description of what the Framers unquestionably did in Article II of the Constitution," he said Friday. "If Congress attempts to vest the power to execute the law in someone beyond the control of the President, it contravenes the Framers' clear intent to vest that power in a single person, the President."

Barr's forceful defense of Trump comes as some of the president's allies grumble that he has failed to publicly back the White house during the impeachment investigation, particularly after a report surfaced that Barr turned down a request from Trump to hold a press conference saying no wrongdoing occurred in the president's dealings with Ukraine, the focus of the House's impeachment investigation.