Private tensions between U.S. Justice Department leaders and special counsel Robert Mueller's team broke into public view in extraordinary fashion before the Senate judiciary committee Wednesday as Attorney General William Barr pushed back at what he called the special counsel's "snitty" complaints over his handling of the Trump-Russia investigation report.

Testifying for the first time since releasing Mueller's report, Barr faced sharp questioning from Senate Democrats who accused him of making misleading comments and seeming at times to be U.S. President Donald Trump's protector as much as the country's top law enforcement official.

The rift fuelled allegations that Barr has spun Mueller's findings in Trump's favour and understated the gravity of Trump's behaviour. The dispute is certain to persist, as Democrats push to give Mueller a chance to answer Barr's testimony with his own later this month.

Barr separately informed the House Judiciary Committee later Wednesday that he would not appear for its scheduled hearing on Thursday because of the panel's insistence that he be questioned by committee lawyers as well as lawmakers. That refusal sets the stage for Barr to possibly be held in contempt of Congress.

Committee chair Lindsey Graham of South Carolina, right, seen with Republican colleague Chuck Grassley of Iowa, defended Mueller's integrity but also took aim at the FBI agent who was removed from the special counsel team. (Andrew Harnik/Associated Press)

His appearance Wednesday gave Barr his most extensive opportunity yet to explain the department's actions, including his news conference held before the Mueller report's release.

It was also a chance for him to repair a reputation bruised by allegations that he's the Republican president's protector and by the emergence of a private letter from Mueller, released publicly before the testimony, that criticized his handling of the report.

In his letter, Mueller raised concerns about a letter that Barr sent to Congress detailing what he said were Mueller's principal conclusions.

BREAKING: Letter from Special Counsel Robert Mueller to Attorney General Barr. <a href="https://t.co/oDJm6coP8G">pic.twitter.com/oDJm6coP8G</a> —@HouseJudiciary

Mueller said Barr's letter "did not fully capture the context, nature and substance" of the special counsel's work and conclusions.

Barr said he was perturbed by the letter, considering Mueller was a longtime friend. The attorney general called the note "a bit snitty."

"I said 'Bob, what's with the letter? Just pick up the phone and call me if there is an issue,"' Barr said.

The revelation that Mueller, who'd been publicly silent for the entire investigation, was agitated enough to send a letter to Barr — which could, and did, become public — lent his words extra credibility with Democrats, who accused Barr of lying under oath last month when he denied that Mueller's team was unhappy with how their work had been characterized.

House Democrats are trying to get former FBI director Robert Mueller, who oversaw the special counsel probe, to testify. (Cliff Owen/Associated Press)

"There is new public confusion about critical aspects of the results of our investigation," Mueller said in the letter. "This threatens to undermine a central purpose for which the department appointed the special counsel: to assure full public confidence in the outcome of the investigations.

The Democrat-led House judiciary committee is nearing a date on which Mueller will testify, likely sometime in May, committee chair Jerry Nadler of New York told reporters.

Barr said he had no objection to the possibility of Mueller testifying to Congress.

'High state of agitation'

Barr testified Mueller finished his investigation without interference and that neither he nor any other Justice Department official overruled any proposed action. He reiterated that he felt it necessary to issue a four-page statement of "bottom line" conclusions of the Mueller report.

"The body politic was in a high state of agitation," he said.

The Republican-led Senate committee on Wednesday was chaired by Sen. Lindsey Graham of South Carolina. Graham, an ally of the president, nevertheless pushed back on the subject of Russian interference, mimicking a line Trump used in a 2016 presidential debate: "It wasn't some 400-pound guy sitting somewhere."

Trump has continually assailed the probe and even accused Mueller of bias, but Graham in his opening statement praised the thoroughness of the investigation and the former FBI director's integrity.

Graham also highlighted what he characterized as extensive White House co-operation with Mueller's investigators, an assertion many Democrats dispute, given that Trump did not sit down for an in-person interview and instead submitted written answers to questions. As well, he asserted the report made it clear Trump did not obstruct justice.

'That's not a crime'

Under questioning from Graham, Barr agreed there was no obstruction by Trump because there was no "underlying crime," and because the president didn't exert executive privilege over documents requested by Mueller.

"We do not think in this case the government could show corrupt intent beyond reasonable doubt," he said at another point, under questioning from Democratic Sen. Dianne Feinstein of California.

Feinstein focused on a section of the report in which it was alleged that Trump tried to influence White House counsel Don McGahn to change testimony he gave to the special counsel.

"You have a situation where the president tries to change a lawyer's account in order to prevent further criticism of himself," she said.

"That's not a crime," Barr responded.

Watch that exchange:

U.S. Attorney General William Barr appeared before the Senate's judiciary committee to discuss special counsel Robert Mueller's Russia report. 1:07

Some Democrats moved to exploit the daylight between Barr and Mueller to impugn the attorney general's credibility. Some also called for Barr to resign, or to recuse himself from U.S. Justice Department investigations that have been spun off from Mueller's probe.

"I think the American public can see quite well that you are biased in this situation and you have not been objective and that would arguably be a conflict of interest," said Senator Kamala Harris, one of the Democratic contenders for president.

Sen. Kamala Harris, D-Calif., speaks as Attorney General William Barr testifies during a Senate Judiciary Committee hearing on Capitol Hill in Washington, Wednesday, May 1, 2019, on the Mueller Report. (Andrew Harnik/Associated Press)

Graham hit on familiar criticisms that the FBI's Russia investigation was tainted by law enforcement bias against Trump. Graham was the first of a few Republicans who read aloud derogatory texts by agent Peter Strzok, who was assigned to the special counsel team but removed weeks later.

Strzok, at an earlier time, was also a part of the investigation into the use of Trump rival Hillary Clinton of a so-called homebrew server system at her home while serving as secretary of state. Graham blasted the handling of that particular investigation in his statement.

You are no different than Rudy Giuliani or Kellyanne Conway or any of the other people who sacrificed their once decent reputations for the liar and grifter who sits in the Oval Office. - Sen. Mazie Hirono

Graham rarely intervened as the senators took their turns questioning Barr, but was angered after Hawaii Democrat Mazie Hirono called on the attorney general to resign with some harsh criticism.

"Now the American people know that you are no different from Rudy Giuliani or Kellyanne Conway or any of the other people who sacrificed their once decent reputation for the grifter and liar who sits in the Oval Office," Hirono said.

"You have slandered this man from top to bottom," Graham said as Hirono's time expired.

Barr pressed on 'spying' characterization

Barr was only confirmed for his second stint as attorney general in February, succeeding Jeff Sessions, who Trump continually maligned publicly for recusing himself from overseeing the Russia probe.

In his letter in March, Barr revealed that he and deputy attorney general Rod Rosenstein had cleared Trump of obstruction of justice after Mueller and his team found evidence on both sides of the question but didn't reach a conclusion.

After the letter's release, Barr raised eyebrows anew when he told a congressional committee he believed the Trump campaign had been spied on, a common talking point of the president and his supporters.

Sen. Sheldon Whitehouse of Rhode Island grilled William Barr on his previously stated concern that the Trump campaign had been 'spied' on. (J. Scott Applewhite/Associated Press)

Barr told Democrat Sheldon Whitehouse of Rhode Island on Wednesday that when he used the term spying, it was an off-the-cuff characterization.

In April 10 testimony before a House panel, Barr stated, "I think spying on a political campaign is a big deal."

AG Barr is a disgrace, and his alarming efforts to suppress the Mueller report show that he's not a credible head of federal law enforcement. He should resign—and based on the actual facts in the Mueller report, Congress should begin impeachment proceedings against the President. —@ewarren

Barr was invited to appear Thursday before the Democratic-led House judiciary panel. Late Wednesday evening, a U.S. Justice Department spokesperson confirmed Barr would not appear, saying committee chair Jerry Nadler placed conditions of the hearing that were "unprecedented and unnecessary."

A committee subpoena deadline for Barr's department to hand over an unredacted version of Mueller's report on his investigation also expired on Wednesday.

"The Department of Justice has also told us that they will not comply with our subpoena for the full unredacted Mueller report," Nadler said.

Other highlights Wednesday