She also criticized Barr for not committing to publicly releasing Mueller's findings.
Meanwhile, Barr pledged during his hearing to release "as much as I can" of the special counsel findings.
Barr is virtually assured of approval in the Senate, where Trump's Republican Party holds a majority.
Special counsel Robert Mueller is looking into whether the Trump campaign coordinated with Russian Federation and whether the president obstructed the investigation. Dianne Feinstein, D-California, called a memo in which Barr was critical of Mueller's probe "disqualifying", saying she's anxious that he has not committed to releasing Mueller's report to Congress.
A corporate lawyer who previously served as attorney general under Republican President George HW Bush in the early 1990s, Barr has been praised by politicians from both parties as someone who is deeply familiar with the workings of the Justice Department and does not owe his career to Trump. Barr has said he will be as transparent as possible under Justice Department regulations, but many Democrats are skeptical. Charles E. Grassley (R-Iowa) noted at Thursday's hearing he had introduced the bill with Blumenthal that would require the special counsel to produce a report to Congress when his work was done. "We are already seeing unsafe threats to the rule of law".
But Durbin said Barr has "telegraphed his pitch" by writing an unsolicited memo to Justice Department and Trump lawyers a year ago about parts of the special counsel probe.
Democrats on the U.S. House of Representatives' Judiciary Committee on Thursday prepared a subpoena to compel Whitaker to testify on Friday, prompting him to say he will not appear unless the subpoena threat is lifted.
Sen. Richard J. Durbin (D-Ill.) said he felt Barr was a "good person" and likened him to Jim Mattis - Trump's former defense secretary, who had a reputation for being independent and resigned from the administration amid disagreements with the president.
"I don't believe Mr. Mueller would be involved in a witch hunt", Barr said.