Hawaii Senator Mazie Hirono called on Barr to resign at the hearing to review Mueller's report.
Hirono launched an aggressive line of questioning against Barr, asserting he hadn't been honest with Congress and calling on him to resign.
Hirono also asked Barr if it was okay for a president to ask one of his aides to lie, referencing the report's examination of whether Trump obstructed justice.
When Barr equivocated, Hirono grew angry, saying, "Mr Attorney-General, please give us some credit for knowing what the hell is going on right now."
Senate Judiciary Chairman Lindsey Graham shot back: "You have slandered this man from top to bottom."
Barr himself chimed in, asking "How did we get to this point?"
Graham set the tone for his party at the start saying: "For me, it is over.".
Gavelling open the session, Graham outlined the scope of Mueller's nearly two-year probe, with its 2000 subpoenas and million-plus documents turned over by the White House, and gave his take on the findings.
"So, no collusion, no coordination, no conspiracy," Graham said.
Barr described a letter from Mueller expressing concerns about his portrayal of the Russia probe as "snitty".
Democrat Richard Blumenthal asked about the letter. Mueller wrote it March 27, but it was only disclosed publicly ahead of the hearing.
"The letter's a bit snitty," Barr said. He said he thinks it was probably written by someone on Mueller's staff.
Barr said he called Mueller the next day and said: "What's with the letter? Why don't you just pick up the phone and call me if there was an issue?"
Blumenthal characterised the letter an "extraordinary act" of "rebuking the Attorney-General of the United States" and "memorialising it in writing."
The House Judiciary Committee has voted to allow its staff to question Barr, throwing his scheduled testimony tomorrow into question.
The Democrat-led panel voted to allow extra time for questioning. Barr has objected to the change.
It's unclear whether Barr will testify before Chairman Jerrold Nadler's panel as scheduled.
Nadler speculated that Barr "is afraid" of testifying, adding, "he apparently does not want to answer questions."
Republicans shot back that Democrats are conducting impeachment-like proceedings against Trump instead of legitimate oversight.
- AP
