"I will not be bullied into doing anything that I think is wrong, whether it be by editorial boards, Congress or the president", Barr told senators on the Judiciary Committee.
William Barr, President Trump's pick for attorney general, faces the first day of confirmation hearings on Capitol Hill Tuesday morning.
Sen. Dick Durbin, of IL, on ABC's "This Week" said he wants "ironclad assurances" of Barr's independence and willingess to let Mueller finish his probe.
Barr will tell senators the Justice Department can not allow people to flout America's legal system by "crashing in through the back door".
"It's unimaginable to me that Bob would ever do anything that gave rise to good cause", Barr said.
"The memo did not address - or in any way question - the Special Counsel's core investigation into Russian interference in the 2016 election", Barr will say, according to the AP.
Barr is expected to win confirmation in the Republican-controlled Senate to serve as the nation's top law enforcement officer, which would put him in charge of Special Counsel Robert Mueller's investigation into Russia's interference in the 2016 election and any possible collusion between Moscow and Trump's campaign.
He repeatedly said special counsel Robert Mueller must be allowed to complete his inquiry into whether the Trump campaign colluded with an alleged Russian plot to influence the 2016 United States presidential election.
He has come under criticism from Democrats for the memo, which he sent Deputy Attorney General Rod Rosenstein. Barr, as head of the Justice Department, would take over from acting attorney general Matthew Whitaker and oversee Mueller's work. He has even said that Trump made the "right call" when he fired Federal Bureau of Investigation director James Comey, in a move that was criticized not just by Democrats but also by Republicans.
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Barr as a private citizen penned a June memo that went after the Mueller probe.
He said he had written a similar memo criticising the department's corruption case against Democratic Senator Bob Menendez, which ended in a mistrial in 2017.
[Trump] wanted to know what I thought about Mueller's integrity and so forth and so on. In addition to discussing his memo with Trump's personal attorneys, Barr also shared or discussed it with Kushner's lawyer.
Barr stated without hesitation that it was in the public interest for Mueller to finish his investigation into whether the Trump campaign coordinated with the Kremlin to sway the election.
The special counsel is required to report his findings confidentially to the Justice Department.
Mr Barr stopped short of promising to release Mr Mueller's report.
Karl said, "We are now just, I believe, weeks away from getting a final report from Robert Mueller, and there is so much anticipation, so much speculation about what he is going to show". He told The Hill previous year that "leaks" from the special counsel's office "raise questions as to whether there is an agenda" and suggested that Mueller's team was not politically "balanced," as some members had made political contributions to Democratic candidates. It is not clear how much of the investigation will be left by then.
Since Jeff Sessions had recused himself from the Russian Federation investigation due to his own ties to the Trump campaign, Deputy Attorney General Rod Rosenstein was acting Attorney General when it came to the investigation.
During the hearing, Barr's past comments about the Mueller investigation attracted scrutiny, including an unsolicited memo he sent the Justice Department a year ago criticizing the special counsel's inquiry into whether Trump had sought to obstruct justice. He said presidents can not be criminally investigated for actions they are permitted to take under the Constitution, such as firing officials who work for them, just because of a subjective determination that they may have had a corrupt state of mind.
Barr said in his prepared testimony he meant to "keep up the pressure" on chronic violent criminals; focus on the danger of political violence in what he called a highly divided nation; and execute the administration's priorities on border and immigration enforcement.