Washington (CNN) Attorney general nominee William Barr's Senate confirmation hearing Tuesday is expected to be a highly charged affair that's sure to test his opinion of just how powerful a president -- in this case, Donald Trump -- is.

If Barr wins confirmation, he'll be thrust into the lightning rod role of overseeing the finale of the gravest probe in decades into a sitting President who has a long record of chafing against legal norms and constitutional restraints.

The hearing, which was always guaranteed to be politically volatile, has taken on even more significance following stunning new revelations about Trump's mysterious relationship with Russia -- including a New York Times report that the FBI opened an investigation into whether the President was working for Moscow.

It will likely fall to Barr to decide how special counsel Robert Mueller's inquiries will end , what will happen to his final report and how much the public will find out about it. Skeptics of Barr's nomination will be seeking to secure assurances that he will do as much to shield Mueller as Deputy Attorney General Rod Rosenstein, who has defied Trump's wrath but is expected to step down shortly after Barr arrives.

"There are good reasons to be concerned about the rule of law, and the independence of the judiciary and the rule of the Department of Justice," said Sen. Chris Coons, D-Delaware, previewing the hearing on a conference call Monday with reporters.

"We have a President who almost daily criticizes the FBI and the Department of Justice, and in particular the Mueller investigation. We have a President whose campaign manager, national security adviser and personal attorney have all either pled guilty to and been charged with or convicted of crimes related to their conduct, either on the campaign or in the support of President Trump and his business activities," he said.

"That is not a typical or common context in which to have a confirmation hearing about an attorney general," Coons added.

While Barr is not to blame for the political storm raging around the hearing, his own beliefs and paper trail over a long legal career are also sparking controversy.

His belief in a contentious doctrine of strong executive power and his doubts about the viability of charging a president with obstruction of justice have put him on a collision course with many Trump critics, despite respect for his credentials for a second run as attorney general.

Every attorney general must answer the conundrum of how to balance a White House's political goals and actions with tensions that sometimes arise and challenge the rule of law and the independence of the Justice Department.

For Barr, those questions may be more extreme than for any recent predecessor, at least since hotly contested legal determinations that were made about the treatment of terror suspects and executive power in the dark days after 9/11.

In two years as President, Trump has almost constantly tested the norms of Washington and the assumed limits of his office. He excoriated his former attorney general, Jeff Sessions , over his recusal from the Russia investigation and has pounded the Justice Department and the FBI over what he sees as a "witch hunt." He frequently rails against judges who block his goals.

He demands loyalty from his subordinates, seeming to regard the Justice Department as a personal white-collar law firm that should shield him and pursue his opponents, rather than the body set up to ensure the fair administration of the rule of law and respect for the Constitution.

Such questions will be at the center of the early barrage of questions from opposition Democrats during the hearing on Tuesday morning.

'Witch hunt?'

The question of how the Mueller probe will end was given added relevance by recent indications that the White House will seek to obscure the details of any final report with a series of aggressive executive privilege claims.

In an apparent attempt to defuse some of the most explosive questions, Barr released written testimony on Monday , promising that the special counsel would be allowed to finish his investigation and that the public and Congress deserved to learn the results consistent with the rule of law.

Barr's pledge left plenty of room for interpretation, not least in how much "transparency" is required by the law -- a gray area that Democratic senators will try to color in on Tuesday.

But he also promised that "where judgments are to be made by me, I will make those judgments based solely on the rule of law and will let no personal, political or other improper interests influence my decision."

Barr also noted he had known Mueller for 30 years, that they were friends and that he had welcomed Mueller's appointment.

"I had confidence he would handle the matter properly. I still have that confidence today," he said.

Barr's statement was remarkable given that Trump has spent months trying to discredit Mueller, branding the FBI investigation as recently as Monday as a "whole big fat hoax" and an effort by a Washington "deep state" to invalidate his election.

The President has yet to react to the testimony. But Barr will have known he would risk alienating the President right from the start given his treatment of Sessions and other senior FBI and Justice Department officials.

Still, such a strong early show of independence could also offer Barr, a veteran Washington operator, more leeway to interpret his duty of transparency on the Mueller report in a way that some of Trump's critics might question.

John McGinnis, an assistant attorney general under Barr when he was head of the Office of Legal Counsel in the Justice Department, said he believes Barr's reputation and personal situation as a well-compensated corporate attorney make him an ideal fit for a fraught political moment.

"(He) is in quite a powerful position in the sense that he doesn't really need this job," said McGinnis, now a professor of constitutional law at the Pritzker School at Northwestern University.

"If the President asks him to do something that he believes is illegal, he can resign. In that sense he has some rather substantial leverage in a way that some younger people who are seeking the next job after attorney general (and) don't want to have a reputation of being a troublemaker (aren't) in that position," he said.

"That structure, I would say, should give some confidence that the attorney general isn't going to just do something that he thinks is wrong to get along with the President," McGinnis added.

Testing the guardrails of democracy

Barr's experience and personality may well mean that he will not face the kinds of questions that other Trump administration nominees have had to deal with about their qualifications for the jobs they are seeking.

But many Democrats, already spooked by Trump's frequent scrapes against the guardrails of his office, are concerned about past writings by Barr that express strong support for almost unfettered executive power.

This is especially worrying for critics who fear that Trump's ambition is to place himself above the law and that he would be better served by an attorney general with a more limited interpretation of executive power.

Concern about Barr among Democrats spiked over an unsolicited memo he wrote last June to the Justice Department arguing that Mueller "should not be permitted to demand that the President submit to interrogation about alleged obstruction."

The fact that Barr admitted in the memo that he was "in the dark about many facts" led some critics to see his arguments as a political move that could have been motivated by a desire to promote himself for a return to Justice, where he served President George H.W. Bush as attorney general.

In his testimony and in a letter to Sen. Dianne Feinstein of California, the top Democrat on the Judiciary Committee, Barr sought to head off controversy about the memo.

He pointed out that he was arguing a specific theory on a single statute that he thought Mueller might be considering.

"The memo did not address -- or in any way question --the Special Counsel's core investigation into Russian interference in the 2016 election," Barr wrote in his testimony.

"Nor did it address other potential obstruction of justice theories or argue, as some have erroneously suggested, that a President can never obstruct justice," he added.

Tuesday's hearing will be the first chance for the Senate Judiciary Committee to grab a national public spotlight since it fragmented into bitter acrimony over the confirmation of Supreme Court Justice Brett Kavanaugh last year.

It will also provide another stage for potential 2020 Democratic presidential candidates, such as Sens. Cory Booker of New Jersey, Amy Klobuchar of Minnesota and Kamala Harris of California, to seize the opportunity to impress liberal activists.

Barr's tough views on criminal justice -- an issue in which legal and political thought have eased since he last served as attorney general -- intersect directly with a key issue for Democratic candidates.

His positions on civil rights, LGBTQ equality and immigration -- which are consistent with those of a conservative Republican of the 1980s and 1990s -- could also provoke explosive exchanges with Democrats on issues that are driving their party's transformation.