Mueller testimony expected to heighten scrutiny of perceived conflicts with Attorney General Barr

Robert Mueller's highly anticipated testimony before Congress Wednesday is expected to put renewed scrutiny on Attorney General Bill Barr, whose controversial handling of the special counsel's final report and repeated critiques targeting the origin of the Russia investigation have sparked furor among Democratic lawmakers.

Mueller's delivery of his report in late March immediately put a microscope on Barr just over a month into his tenure as attorney general, and his decision just days later to clear the president of any wrongdoing regarding potential obstruction of justice -- compounded by his repeated public defenses of the president -- has only served to fuel suspicions of his motives.

Democrats are expected to use Mueller’s testimony to highlight what they argue are discrepancies between what they see as damning findings in Mueller’s report paired with Barr’s public statements seeming to downplay Trump’s behavior in response to the special counsel’s investigation.

"As [Trump] said from the beginning, there was in fact no collusion," Barr said in an April 18 news conference announcing the release of the report. "And as the Special Counsel’s report acknowledges, there is substantial evidence to show that the President was frustrated and angered by a sincere belief that the investigation was undermining his presidency, propelled by his political opponents, and fueled by illegal leaks."

It’s not clear, though, whether Mueller will engage in questioning that seeks to undermine Barr’s statements, as Mueller has pointed to his own report as sufficient testimony and expressed confidence in Barr’s decision to release it to the public.

"The Attorney General preferred to make that — preferred to make the entire report public all at once, and we appreciate that the Attorney General made the report largely public," Mueller said in a May 29 news conference. "And I certainly do not question the Attorney General’s good faith in that decision."

And in a letter to Mueller Monday sent as a response to Mueller seeking guidance about the boundaries of his testimony, Associate Deputy Attorney General Bradley Weinsheimer wrote that all his statements "must remain within the boundaries of your public report because matters within the scope of your investigation were covered by executive privilege."

Did Barr and Mueller split over obstruction case?

Democrats initially sought to describe Mueller's news conference in May as a "direct rebuke" to statements make by Attorney General Barr about the role that a long-standing DOJ policy played in Mueller's decision not to say whether President Trump obstructed the Russia investigation.

While Barr and Mueller seem to agree that Mueller never was capable of charging President Trump with a crime, Barr has said he disagreed with Mueller over whether he could publicly state that Trump's conduct was criminal in nature.

"We concluded that we would not reach a determination – one way or the other – about whether the President committed a crime,” Mueller said.

But in an interview with CBS News, Barr said he "personally felt [Mueller] could've reached a decision."

"The opinion says you cannot indict a president while he is in office but he could've reached a decision as to whether it was criminal activity," Barr said. "But he had his reasons for not doing it, which he explained and I am not going to, you know, argue about those reasons."

“We don’t think Director Mueller is going to change views about what the OLC policy says he can or cannot do despite the Attorney General taking an obviously very different view," a senior House Judiciary committee staffer told ABC News. "But we do think he’s going to lean into his factual findings that his team made as well as the legal positions.”

Asked in a recent New York Times interview why he didn't simply direct Mueller to make a determination, Barr answered that he, "wasn’t going to try to bully him into doing something different."

Barr has also sought to distance the DOJ from Mueller's decision to state that if his team "had confidence that the president clearly did not commit a crime, we would have said so."

"[That] of course is not the standard we use at the department," Barr said in an interview following Mueller's news conference. "We have to determine whether there is clear violation of the law and so we applied the standards we would normally apply. We analyzed the law and the facts and a group of us spent a lot of time doing that and determined that both as a matter of law, many of the instances would not amount to obstruction."

In interviews since the release of the Mueller report, Barr has issued defenses of several of the specific episodes of potential obstruction outlined in Mueller's report.

He has said that Trump's firing of former FBI Director James Comey in May 2017 would likely not amount to an obstructive act as it would be difficult to prove beyond a reasonable doubt that the firing impeded the Russia investigation, and that Trump had a "corrupt intent" in ordering Comey's removal.

Barr has also cast doubt over whether Trump ordering former White House counsel Don McGahn to facilitate Mueller's removal as special counsel would have counted as obstruction, because at the time Barr says Trump was venting about what he described as "conflicts" that Mueller may have had.

Though in the special counsel's report, advisers such as Steve Bannon are quoted directly telling the president at the time that his conflict claims were "ridiculous" and "did not count as true conflicts."

The 'snitty' letter: Was Mueller upset with how Barr initially characterized his report's findings?

Another key issue likely to surface in Wednesday's hearings is a letter Mueller penned to Barr in the days following the release of Barr's 'principal conclusions' letter outlining what he saw as the top line findings from the two and a half year investigation.

In that letter, Mueller expressed discomfort that the letter, "did not fully capture the context, nature, and substance of this Office's work and conclusions."

"There is now public confusion about critical aspects of the results of our investigation," Mueller wrote. "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."

In an appearance before Congress on May 1, Barr sought to downplay the apparent concerns expressed by Mueller by detailing a follow-up phone conversation the two had following his receipt of Mueller's letter.

"I can’t speak to the team as a whole, but certainly when I talked to Bob Mueller he indicated he was concerned about the press coverage that had gone on the previous few days and he felt that was to be remedied by putting out more information," Barr said. "He felt that what was inaccurate was the press coverage and what they were interpreting the March 24 letter to say."

Ultimately, however, Barr said he "made it clear" to Mueller on the call that he would not be putting out executive summaries the special counsel's office had incorporated in the report.

"A summary would start a whole public debate, it’s by definition underinclusive and I thought what we should do is focus on getting the full report out as quickly as possible, which we did," Barr said.

That Mueller would send such a letter, though, seemed to irk Barr, as he used it to take a jab at Mueller's deputies near the close of the hearing.

"The letter is a bit snitty and I think it was probably written by one of his staff people," Barr said.

Will Mueller split with Barr on criticism of the Russia investigation's origins?

Since the release of Mueller's report, Barr has delighted Republicans and the president with his renewed push to discover whether there was any corrupt motive among investigators who initially started the investigation of Russia's efforts to interfere in the 2016 election.

Barr has said he has thus far been given "inadequate" answers regarding the surveillance of members of Trump's campaign who were found to be in contact with Russians during the campaign, and Barr further rankled intelligence officials when he described the FBI's tactics as "spying."

"People have to find out what the government was doing during that period," Barr told Fox News in an interview. "If we're worried about foreign influence, for the very same reason we should be worried about whether government officials abuse their power and put their thumb on the scale."

Mueller, on the other hand, stayed silent through his two and a half year investigation as Trump and his allies ramped up their attacks on his investigative team, issuing no public statements in response to concerns that he was mounting a 'witch hunt' against Trump with a team of politically motivated investigators.

Though Mueller did not directly address such criticism in his news conference, he did issue a defense of both the investigation of Russia's activities and those who sought to undermine or impede it.

"The matters we investigated were of paramount importance and it was critical for us to obtain full and accurate information from every person we questioned," Barr said. "When a subject of an investigation obstructs that investigation or lies to investigators, it strikes at the core of the government’s effort to find the truth and hold wrong doers accountable."

ABC News' Katherine Faulders contributed to this report.