Two months after Donald Trump fired his original attorney general, the Senate will on Tuesday cross-examine his proposed replacement, William Barr, a Washington stalwart who served in the role under George HW Bush.

The pick has been welcomed by Republicans and even some Trump detractors, who reason that Barr is a known quantity and that Trump might have done much worse, given his extraordinary views on presidential impunity and his expressed hostility toward the Department of Justice (DoJ).

But some Democrats, former federal prosecutors and DoJ figures are warning that Barr might not be such a known quantity after all, and that confirming him as attorney general would threaten the work of the special counsel Robert Mueller and the justice department more broadly.

Barr’s nomination has surfaced at an unusually fraught moment in Washington. While clashes between the White House and justice department are not unheard of, Trump’s war on the former attorney general Jeff Sessions, his attacks on top justice department figures such as the deputy attorney general, Rod Rosenstein, and former FBI director, James Comey, and his stated belief that he should be able to direct justice department investigations – all combined with the sheer magnitude of alleged criminal conduct by former Trump aides and business associates and possibly Trump himself – means the next attorney general will wield power with likely historic consequences.

His view is outside the mainstream even of conservative, basic unitary executive theory Neil Kinkopf

The announcement last week of the planned departure of Rosenstein, who is seen as having faithfully protected the Mueller investigation against serious pressure from the White House, ratcheted stakes even higher.

Critics of Barr have raised an alarm about a memo that Barr submitted to the department last year, ostensibly unsolicited, arguing that Mueller’s investigation of Trump for alleged obstruction of justice was “fatally misconceived”.

“The new Barr memo makes it clear that his view is outside the mainstream even of conservative, basic unitary executive theory,” said Neil Kinkopf, a Georgia State law professor who worked in the Office of Legal Counsel during the Clinton administration. “It goes beyond that, to the point where it comes very close to putting the president above the law.”

Senator Richard Blumenthal of Connecticut, a Democrat on the judiciary committee, which will question Barr and vote on his nomination, said: “The memorandum is deeply worrisome because in effect he says the president is above the law.

“That’s incorrect as a matter of law, but certainly for an attorney general to have that position is deeply wrong.”

Other critics have raised concerns that Barr, who as attorney general in the early 1990s recommended that Bush pardon six Reagan officials caught up in the Iran-Contra affair, was nominated to defend the White House, not just to run the justice department.

These critics point to Barr’s recent record as a Trump apologist. After the firing of Comey in 2017, Barr published an op-ed in the Washington Post praising the move. He also has supported Trump’s call for a criminal investigation of Hillary Clinton’s ties to a uranium mining company that benefited from a decision made while she was secretary of state, a marginal view mainly consigned to the fever swamps of the far right.

Count me as one Democrat who thinks President Trump has made an excellent choice Harry Litman

Barr, 68, grew up in New York City and graduated from George Washington University law school before taking up a post on Ronald Reagan’s domestic policy staff. At the justice department he served in the Office of Legal Counsel, as deputy attorney general and finally, from 1991 to 1993, as attorney general, spotlighting gang violence and prison conditions – Barr argued that judges were dictating unrealistic quality-of-life standards.

Barr – who is in his spare time an expert bagpiper – was also an immigration hardliner, hiring hundreds of new border guards and, according to a 1992 CBS News report that called him “a tough guy who’s not afraid to ruffle feathers”, swearing in some of them himself “to underscore he meant business”.

A 1991 Los Angeles Times report noted Barr’s “dry, self-deprecating wit and full-bodied laugh” but called him “a staunch conservative who rarely hesitates to put his hardline views into action”, citing a 1989 legal opinion in which Barr authorized federal agents to seize fugitives overseas without a foreign government’s permission.

William Barr with President George HW Bush, in 1991. Photograph: Scott Applewhite/AP

However, even some strong Trump critics believe that, based on his past career, Barr would reprise his leadership of the justice department in good faith and not with undue deference to the White House.

“Count me as one Democrat who thinks President Trump has made an excellent choice in his decision to nominate William P Barr,” Harry Litman, a former prosecutor who worked under Barr, wrote in December.

“Was he, and is he, very conservative? You bet … But the roots of his political views concern the rule of law, public safety and the fair application of legal rules to all.”

But others believe that Barr’s memo criticizing Mueller is an indication that his judgment or perspective might have changed in the 25 years since he was known for steering the justice department with a fair hand.

“That reputation would make him all the more effective an attack dog for Trump,” said Kinkopf, who has argued elsewhere that Barr’s “expansive view of presidential power” laid the legal groundwork for the George W Bush-era torture program.

“If he has gone over the to the dark side, so to speak, his past credible service makes him all the more effective in undermining the rule of law, and all the more dangerous.”