Mr Barr has long been a lightning rod of anger and frustration from Democrats for his handling of the Mueller report, his heavy-handed response to racial justice protests this past summer, and his attempts to investigate FBI and DOJ officials who led probes into Mr Trump’s 2016 campaign.
Recently, the attorney general has become something of a pariah on the right for defying Mr Trump’s claims of election fraud.
Virginia Democratic Congressman Don Beyer summed up his party’s position on the outgoing attorney general with a tweet shortly after the president announced his imminent departure.
“I don't know whether Bill Barr is actually resigning or being fired, but either way he will go down in history as one of the worst Attorneys General in American history,” Mr Beyer said.
And Connecticut Democratic Senator Richard Blumenthal embodied Democrats’ feelings of animus to Mr Barr.
“There should be no sympathy or regret for an Attorney General who trashed the rule of law, caused untold suffering, & enabled a morally depraved president. Don’t let the door hit you on the way out,” Mr Blumenthal tweeted.
Congressman Ted Lieu, who sits on the House Judiciary Committee that has had its battles with Mr Barr over the last two years, accused the attorney general of turning the DOJ into “Trump’s personal law firm.”
Mr Trump wished the attorney general well and said he would be replaced in an acting capacity by Deputy Attorney General Jeff Rosen.
Theories on the timing Mr Barr’s resignation abounded on the internet, with some pundits speculating Mr Trump timed it to distract from President-elect Joe Biden’s Electoral College victory.
“Donald Trump fired Bill Barr just after Biden was formally elected by the Electoral College to CHANGE THE NEWS CYCLE. This has been a pattern of Donald Trump's for many years. He always does something shocking on a day when he is being humiliated. Always. Always. Always,” author Don Winslow tweeted.
Mr Barr’s fall from the president’s good graces underscored just how quickly a trusted hand could garner nothing but praise from the boss one day and be on his naughty list just a few days or weeks later.
Mr Barr’s final misstep came when he admitted the DOJ had not found any credible allegation of widespread voter fraud.
“There’s been one assertion that would be systemic fraud and that would be the claim that machines were programmed essentially to skew the election results. And the DHS and DOJ have looked into that, and so far, we haven’t seen anything to substantiate that,” Mr Barr told the Associated Press, also referring to the Department of Homeland Security.
Justice officials have received complaints about some shenanigans – just as they do every cycle.
"Voter fraud claims that had been submitted to the country’s top law enforcement department were “very particularized to a particular set of circumstances or actors or conduct,” Mr Barr told the wire service.
Mr Trump’s inner circle and conservative pundits have also railed against Mr Barr in recent days for not publicly announcing before the election that the DOJ had been investigating Hunter Biden, the president-elect’s son. Doing so, they believe, would have legitimised the conservative mud-slinging against the former vice president’s son and put Mr Biden in an even more difficult political position to answer for his behaviour.
For most of his term as AG, Mr Barr appeared to be one of Mr Trump’s favourites.
They shared a vision of the Office of the President having vast powers, and seemed to have a sort of agreement to help one another codify as much of the “unitary executive” theory as they could.
Mr Barr misled the media about the contents of former special counsel Robert Mueller’s report on his years-long investigation into ties between Trumpworld and Russia and whether the president sought to obstruct his probe.
The attorney general backed the president’s claims about the potential hazards of mail-in voting and took aggressive action to tamp down violent pockets of racial justice protesters throughout US cities this past summer, leading to several high-profile showdowns with local elected officials, such as the mayors of Portland, Oregon, and Chicago.
And Mr Barr took several administrative actions to affect ongoing investigations and prosecutions into political allies of Mr Trump, including former Trump national security adviser Michael Flynn and former Trump campaign adviser Roger Stone.
GQ magazine correspondent and foreign policy writer Julia Ioffe had a parting query for Mr Barr.
“I have one question for Bill Barr: was it worth it?” Ms Ioffe tweeted on Monday.
Mr Rosen will replace Mr Barr on 23 December.
John T Bennett contributed to this report.
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