Virginia Sen. Mark Warner, the top Democrat in the Senate Intelligence Committee, questioned Attorney General's William Barr credibility to oversee a Justice Department probe into the origins of the Russia investigation, accusing him of acting like President Trump's "personal advocate."

"Mr. Barr has very little credibility with me and I think the vast majority of, candidly, not just Democrats, but many Americans because he time and again is not acting as our attorney general, but as a personal advocate for Donald Trump," Warner said on "Face the Nation" Sunday.

After multiple calls by Mr. Trump to "investigate the investigators," the attorney general last month tapped the U.S. attorney in Connecticut, John Durham, to lead an investigation into the origins of the inquiry into Russian meddling in U.S. elections and possible coordination between the Trump campaign and Moscow. The president recently granted Barr sweeping authority to declassify counterintelligence information and instructed the nation's intelligence agencies to cooperate with the attorney general's review.

In an exclusive interview with CBS News last week, the attorney general said he does not think some Obama-era Justice Department officials who oversaw the Russia investigation committed treason "as a legal matter," but noted he nevertheless has some concerns about the way these officials handled the probe.

"Sometimes people can convince themselves that what they're doing is in the higher interest and better good," Barr added. "They don't realize that what they're doing is really antithetical to the democratic system that we have."

Warner said Barr's review — the third to look into the Russia probe, possible bias among Justice Department officials and whether government surveillance of the Trump campaign was appropriate — will ultimately vindicate officials who launched and oversaw the initial stages of the investigation, which concluded when special counsel Robert Mueller submitted his report in March.

"Have at it because facts will confirm what the intelligence community and law enforcement did was right," Warner said, adding that it would've been "irresponsible" for the FBI not to begin an investigation after evidence of Russia's sophisticated election interference campaign surfaced.