Mr. Barr has not made his motivation clear. But in three months as attorney general, he has aligned himself with the president’s dim view of the inquiry. He declined to knock down the notion that the Russia investigation was a witch hunt, described investigative efforts into the Trump campaign as “spying” and begun the multiagency review into the roots of the investigation. Mr. Barr decided that there was insufficient evidence to prosecute Mr. Trump for obstruction of justice, a decision that the special counsel, Robert S. Mueller III, declined to make.

Mr. Barr served as a driving force in securing the power to declassify government secrets, and the lead-up to Thursday’s announcement demonstrated an amount of planning that went beyond previous similar forays by Mr. Trump and his aides. In July, when they announced that they planned to take away the security clearance of the former F.B.I. director James B. Comey, they had not done the homework to discover he no longer had one.

Mr. Barr asked for the White House to grant him additional, far-reaching powers for his review, according to two administration officials. The White House counsel, Pat Cipollone, who was an assistant to Mr. Barr during his first stint as attorney general under President George Bush in the early 1990s, oversaw the effort to grant Mr. Barr’s request, the officials said.

And though the White House was ready last week to release the memo outlining his new powers, Mr. Barr asked for the White House to wait until he had taken care of outstanding business, including telling the heads of all the relevant intelligence agencies about the coming change.

When the White House released the memo on Thursday evening, it landed with authority and a presentation that signaled a concerted effort unlike Mr. Trump’s tweets or stream-of-consciousness comments to reporters. The document was written in legalese and issued as a memorandum on White House letterhead.

For Democrats, Mr. Barr’s newfound powers served as a sign that Mr. Trump had found a new, and potentially effective, tool in his war on the so-called deep state.

“This is a president who will lash out and destroy anything if he believes it will suit his interests,” said Representative Adam B. Schiff, Democrat of California and the chairman of the House Intelligence Committee. “And he now has a capable lieutenant in the attorney general to help him do just that.”