None of this guarantees that Mueller will be able to complete his work unhindered, or that Americans will ever know what work he did. Ominously, Barr, while promising “as much transparency as I can consistent with the law,” suggested he might try to bury the special counsel’s report by treating it as confidential and releasing only “certain information” himself.

William P. Barr, President Trump’s nominee for attorney general, on Capitol Hill on Tuesday. (Andrew Harnik/AP)

Still, Mueller’s de facto affirmation hearing should be of concern to Trump as the president tries to discredit whatever the special prosecutor comes up with in the coming weeks or months. Just about everybody but Trump regards Mueller as an upstanding man doing honest work. Even Trump’s potential new attorney general.

Barr described declining an earlier request to join Trump’s legal defense team, saying, “I didn’t want to stick my head into that meat grinder.” He recalled telling Trump at the time that “Bob is a straight shooter and should be dealt with as such.”

Regarding his “good friend” of three decades, Barr vowed unequivocally: “On my watch, Bob will be allowed to finish his work.” If ordered to fire Mueller without cause, he said, “I would not carry out that instruction.”

And what if Trump’s lawyers attempt to edit the Mueller report, as has been threatened? “That will not happen.” Barr warned that the president’s interference in cases involving himself and his associates could be unconstitutional or criminal. He even qualified his earlier memo criticizing parts of the Mueller investigation, saying, “I had no facts.”

Barr’s appearance seemed to have a calming effect on the panel so recently shredded by the Brett M. Kavanaugh Supreme Court confirmation. It was as though the appearance of the 68-year-old Barr, confirmed by the same committee 27 years ago to serve the same role in President George H.W. Bush’s administration, had transported the lawmakers to a kinder, gentler time. Instead of trading barbs, Democrats and Republicans took turns talking about the nominee’s grandson. (The 8-year-old’s “Dear Grandpa” note to the nominee mid-hearing was a hit.)

Maybe the bombshell reports over the weekend about Trump’s Russia ties had cowed the Republicans. Whatever the cause, they were disinclined to defend Trump. Sen. Charles E. Grassley (R-Iowa) joined Democrats in pushing Barr for an expansive release of the Mueller report, saying, “The taxpayers ought to know what their money was spent for.”

And the chairman, Sen. Lindsey O. Graham (R-S.C.), usually a Trump loyalist, seemed to be trolling the president.

“Do you believe Mr. Mueller would be involved in a witch hunt against anybody?” Graham asked, invoking the president’s favorite phrase.

“I don’t believe Mr. Mueller would be involved in a witch hunt,” the nominee replied.

Asked whether then-Attorney General Jeff Sessions was right to recuse himself from the Russia investigation — a source of Trump’s fury — Barr replied: “I think he probably did the right thing recusing himself.”

“I agree,” Graham added, before poking fun at Trump’s lack of intellectual curiosity. “President Trump is a one-pager kind of guy,” he said.

“I suspect he is,” Barr concurred.

There was laughter in the hearing room at Trump’s expense.

Trump, no doubt encouraged by Barr’s earlier Mueller memo, hopes to be protected by his new attorney general. And it is possible Barr wasn’t being honest in his professed respect for Mueller and for transparency.

But why would Barr come out of retirement, instead of spending “cherished time” with grandchildren, to take a job he already had — only to become a villain for covering up Mueller’s findings?

“You seem like a rational person,” Sen. Richard J. Durbin (D-Ill.) told Barr. “Why do you want this job?”

Indeed, he’s joining a president famous for chewing up once-respected figures and sending them packing in disgrace and humiliation. Trump reportedly referred to Sessions as “Mr. Magoo” and “mentally retarded” and demeaned him publicly.

Barr’s answer to Durbin should have sent chills down the presidential spine as he munched on leftover Big Macs at the White House.

The rule of law, Barr said, “is the heartbeat of this country,” and he vowed to “protect the independence and the reputation of the department.” Trump’s treatment of subordinates “might give me pause if I was 45 or 50 years old, but it doesn’t give me pause right now,” Barr continued. He added, “I will not be bullied into doing anything I think is wrong.”

Barr spent decades building his reputation. Why would he throw it away now by becoming the guy who buried the Mueller report?

Twitter: @Milbank