MARK THIS as another big step in the erosion of standards at Attorney General William P. Barr’s Justice Department.

The department on Tuesday suggested a light sentence for President Trump’s old friend Roger Stone, by overturning a previously filed and tougher proposal. It did so over the strong objections of four career line prosecutors, all of whom resigned from the case; one left the department entirely. This extraordinary intervention played out publicly after Mr. Trump tweeted his displeasure over the initial recommendation that Mr. Stone spend seven to nine years in prison for obstructing Congress and witness tampering, which was in line with the department’s sentencing guidelines.

The Justice Department insists that the decision to reverse course came before the president’s tweet. But senior officials did not need a tweet to conclude that the president would react angrily to a tough sentence for his longtime crony, and to act in anticipation — or fear — of the president’s predictable reaction.

Meanwhile, the line prosecutors’ resignations provide strong evidence that the department’s reversal was unusual and unwarranted. Indeed, given the crimes of which Mr. Stone was convicted — and the fact that he was caught allegedly threatening a witness — it would have been unreasonable for prosecutors to seek the substantially lighter punishment that Justice now appears to favor.

Now the department has lost at least one career civil servant and yet more credibility. Mr. Barr had already, last year, manipulated the release of the Russia investigation’s findings, using his power over how it would be presented to the public to paint Mr. Trump in a positive light that the actual conclusions did not warrant. Now his department has intervened publicly to skew the punishment that one of the bad actors uncovered in that probe, Mr. Stone, will receive.

The most important role of the attorney general is to protect the department from improper political influence, including from the president. Mr. Barr should have ensured that Mr. Stone’s case was handled with strict professionalism, as the career prosecutors sought to do, and shielded them from White House pressure, direct or indirect. To all appearances, he did the opposite. Mr. Trump evidently thinks so: “Congratulations to Attorney General Bill Barr for taking charge of a case that was totally out of control and perhaps should not have even been brought,” he tweeted.

Senior Democrats are calling for a congressional inquiry. That is certainly warranted. What was the attorney general’s understanding of what the Stone sentencing recommendation was going to be? At what point did he form this understanding? With whom? To what extent was that understanding a reflection of Mr. Stone’s relationship with the president — or set in anticipation of the president’s likely reaction to a tough sentence? Why, after line prosecutors went a different route, was the decision deemed so egregious that it must be overturned, prompting obvious questions about the politicization of the Justice Department?

Senate Judiciary Committee Chairman Lindsey O. Graham (R-S.C.) said there would be no special hearing on the matter. Given his newfound role as presidential enabler, that’s no surprise. But it’s not right, either.

Meanwhile, Mr. Barr should reflect on how, under his watch, the department he has served for so long has become so tarnished.

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