Mr. Barr has a “generally mainstream G.O.P. and corporate” reputation, Norman L. Eisen, who served as special counsel for ethics and government overhaul under President Barack Obama, said on Thursday. But he predicted that Mr. Barr would be vigorously vetted because of what he saw as blots on Mr. Barr’s record, including his push for scrutiny of the mining deal, involving a company called Uranium One.

Mr. Barr “has put forward the discredited idea that Hillary Clinton’s role in the Uranium One deal is more worthy of investigation than collusion between Trump and Russia,” Mr. Eisen wrote in a text message. “That is bizarre. And he was involved in the dubious George H.W. Bush end of term pardons that may be a precedent for even more illegitimate ones by Trump.”

Senator Richard Blumenthal, Democrat of Connecticut, said on Friday that Democrats would carefully vet him.

“I will demand that Mr. Barr make a firm and specific commitment to protect the Mueller investigation, operate independently of the White House, and uphold the rule of law,” Mr. Blumental said in a statement. He deserved particular scrutiny, the senator said, “in light of past comments suggesting Mr. Barr was more interested in currying favor with President Trump than objectively and thoughtfully analyzing law and facts.”

A graduate of George Washington University’s law school, Mr. Barr, 68, got his start in the 1970s working for the C.I.A. and later worked in the Reagan White House before leaving for private practice. In 1989, President George Bush appointed him to lead the Justice Department’s powerful Office of Legal Counsel, and later elevated him to deputy attorney general and then attorney general.

After the Bush administration, Mr. Barr spent most of his postgovernment career as the top lawyer for the telecommunications company that became Verizon, from which he retired in 2008. He later joined the Kirkland & Ellis law firm.

In a November 1992 speech, Mr. Barr put forward the ideal of an attorney general whose primary loyalty is to the rule of law, not to the president who appointed him — saying that he must provide “unvarnished, straight-from-the-shoulder legal advice” with no regard to political considerations like what conclusions the White House might prefer.