Trump also ordered the intelligence community to cooperate with Barr. The memo read: "The heads of elements of the intelligence community... and the heads of each department or agency that includes an element of the intelligence community shall promptly provide such assistance and information as the Attorney General may request in connection with that review."
"Today, at the request and recommendation of the Attorney General of the United States, President Donald J. Trump directed the intelligence community to quickly and fully cooperate with the Attorney General’s investigation into surveillance activities during the 2016 Presidential election," White House press secretary Sarah Sanders said in a statement.
"The Attorney General has also been delegated full and complete authority to declassify information pertaining to this investigation, in accordance with the long-established standards for handling classified information. Today’s action will help ensure that all Americans learn the truth about the events that occurred, and the actions that were taken, during the last Presidential election and will restore confidence in our public institutions."
In a Twitter message late Thursday, Trump lawyer Rudy Giuliani defended the president's action.
"The President @realDonaldTrump made a wise decision to let AG Barr on the documents," Giuliani wrote. "I don’t know for sure but I seriously doubt there’s any national security concern but some of it could affect pending investigations. I’m sure AG and DOJ will make a very appropriate decision."
U.S. Rep. Adam Schiff, D-Calif., chairman of the House Intelligence Committee, blasted the move as an attempt to "weaponize law enforcement and classified information."
Trump claims his campaign was the victim of "spying," though the intelligence community has insisted it acted lawfully in following leads in the Russia investigation.
The president had told Fox News earlier in May that he would allow declassification "soon." He elaborated, "I didn’t want to do it originally because I wanted to wait, because I know what they -- you know I’ve seen the way they play. They play very dirty."
Last month, Barr ran into a buzz saw of criticism from Democratic lawmakers and media figures for testifying that “spying did occur” against the Trump campaign in 2016. But despite the backlash, Barr appeared to be referring to intelligence collection that already has been widely reported and confirmed.
Foreign Intelligence Surveillance Act (FISA) warrants against former Trump campaign aide Carter Page are currently the subject of a Justice Department inspector general investigation looking at potential misconduct in the issuance of those warrants. That review also reportedly is scrutinizing the role of an FBI informant who had contacts with Trump advisers in the early stages of the Russia investigation.
The use of the term "spying" as it applies to the FBI's surveillance in 2016 has been fiercely disputed. The New York Times, even as it reported last year on how the FBI sent an informant to speak to campaign advisers amid concerns about suspicious Russia contacts, stated that this was to "investigate" Russia ties and "not to spy."
“I think spying did occur. The question is whether it was adequately predicated,” Barr testified last month, adding that he believed it is his “obligation” to review whether there was misconduct in the original investigation. “Congress is usually very concerned with intelligence agencies and law enforcement agencies staying in their proper lane.”
He added that “spying on a political campaign is a big deal.”
President Trump backed the attorney general's testimony, saying the same day Barr testified last month that he thinks what Barr said "was absolutely true," adding, "There was absolutely spying into my campaign.”
Democrats, though, charged that the testimony indicated Barr was a compromised witness.
House Speaker Nancy Pelosi, D-Calif., told the Associated Press last month that she doesn’t “trust Barr,” but she trusts Special Counsel Robert Mueller. Senate Minority Leader Chuck Schumer, D-N.Y., accused Barr of “peddling conspiracy theories.”
Fox News' Catherine Herridge, John Roberts, Brooke Singman and the Associated Press contributed to this report.