Mr. Trump elicited those fears by denouncing federal prosecutors who had recommended a prison sentence of up to nine years for his longtime friend and political adviser Roger J. Stone Jr. Mr. Barr fanned them by scrapping the recommendation in favor of a far more lenient one, leading the prosecutors to quit the case in protest.

Mr. Barr then took to national television to complain that Mr. Trump’s angry tweets were undermining him and his department’s credibility — a sign to some current and former lawyers that the department’s freedom from political influence is in imminent danger. Their worries are compounded by the fact that people in Mr. Trump’s circle have been mired in so many criminal or ethical scandals that practically any legal action on those cases could be seen through a political lens.

As many of the department lawyers and some recently departed colleagues see it, Mr. Barr has devoted much of his authority and stature to bolster the president since he took office a year ago.

In ever stronger terms, he has attacked the F.B.I.’s investigation into whether the Trump campaign conspired with Russia to influence the 2016 presidential election. He has said it was mounted on “the thinnest of suspicions” and advanced despite a lack of evidence. The special counsel, Robert S. Mueller III, ultimately found insufficient evidence that the president or his advisers engaged in a criminal conspiracy with Russia but documented their openness to Moscow’s sabotage effort.

While he has pledged that the department will not pursue politically motivated investigations, Mr. Barr said this month that he had created an “intake process” for the president’s personal lawyer Rudolph W. Giuliani to forward supposed proof of misconduct in Ukraine. Mr. Giuliani has claimed to have evidence damaging to former Vice President Joseph R. Biden Jr. and his son.