"I will not be bullied into doing anything that I think is wrong by anybody, whether it be editorial boards or Congress or the president", Barr told the hearing.
"My goal will be to provide as much transparency as I can consistent with the law", Barr wrote.
In his prepared testimony, Barr says his memo "was narrow in scope", regarding "a specific obstruction-of-justice theory under a single statute that I thought, based on media reports, the special counsel might be considering".
'I will follow the Special Counsel regulations scrupulously and in good faith, and on my watch, Bob will be allowed to complete his work'.
Democrats worry that Trump's administration may try to undercut the investigation, which has been a frequent target of the president and his allies.
Democrats suggested a critical 20-page memo that Mr Barr wrote past year to the justice department was a "job interview" to impress Mr Trump. Acknowledging that he was "in the dark about many facts", Barr repudiated Mueller's obstruction of justice theory as "fatally misconceived" and "premised on a novel and legally unsupportable reading of the law".
Republican Committee Chairman Lindsey Graham said Sunday that he's "very comfortable" with answers Barr gave him in a private meeting on Mueller's job security.
This is particularly disturbing because Acting Attorney General Matthew Whitaker, who clearly appeared to be appointed in an effort for the president to gain control over the Mueller probe, reportedly refused an ethics official's recommendation to recuse from it.
Mueller is investigating Russia's interference in the 2016 US presidential election, including "any links and/or co-ordination between the Russian government and individuals associated with the campaign of President Donald Trump". Not only did I never work for Russian Federation, I think it's a disgrace that you even asked that question because it's a whole big fat hoax. But the attorney general, not the special counsel, will decide whether all or some of the report will be shared with Congress and the public.
The remarks attempt to address concerns that Trump intends for Barr to act as a firewall between him and the justice department or to sink the Mueller investigation. He said there were other instances in which he had given advice to the Justice Department, such as cautioning against a prosecution of Democratic Senator Robert Menendez.
Barr's views of presidential power could be important as prosecutors and Democrats in the House of Representatives, where they hold the majority, intensify investigations of Trump's personal business practices and his presidency.
Barr is the nominee to replace Jeff Sessions, whom Trump publicly criticized for recusing himself from all Russian Federation inquiries.
Mr Barr said his predecessor "did the right thing".
Trump has refused to rule out pardons for people who have been swept up, including his former campaign chairman, Paul Manafort, who is awaiting sentence after two federal cases previous year.
On immigration, Barr said that reform is "long overdue" and expressed support for Trump's hardline policies, including funding a border wall.
Barr's own statements about the inquiry have suggested that he is far more sympathetic to Trump's criticisms of Mueller than Sessions or Rosenstein. Trump has also openly mused about changes to libel law to make it easier for the subjects of news stories to sue.