And last week, after Mr. Trump latched on to allegations in a contentious Republican memo that the Justice Department and F.B.I. had abused their surveillance powers and called their conduct a “disgrace,” Mr. Sessions offered a meager defense of his lawyers and the institution. “I have great confidence in the men and women of this department,” Mr. Sessions said in a statement after the memo was released. “But no department is perfect.”

Through a spokeswoman, Mr. Sessions declined to comment for this article.

His muted response to Mr. Trump’s remarks stood in stark contrast with the defiant statements made by Christopher A. Wray, the director of the F.B.I. “I am determined to defend your integrity and professionalism every day,” Mr. Wray said in a message to F.B.I. employees. “Talk is cheap; the work you do is what will endure.”

Mr. Sessions’s standing with Mr. Trump has eroded in his year as attorney general. The president criticized his Senate confirmation performance and his decision to remove himself from overseeing the Russia investigation, even musing that he would not have hired Mr. Sessions had he anticipated the recusal. He berated him so severely over Mr. Mueller’s appointment that Mr. Sessions offered his resignation.

That fraught relationship has impaired Mr. Sessions’s ability to act as a bulwark between his lawyers and politics, former career prosecutors said.

“Sessions’s silence is evidence that Trump’s public neutering of anyone close to this investigation is working,” said Paul Pelletier, a Democratic candidate for Congress in Virginia who was a federal prosecutor for nearly three decades. “It is deleterious to the whole criminal justice process.”

Mr. Pelletier said that although the attorneys general with whom he worked, from Edwin Meese III to Eric H. Holder Jr., clashed with their staff, they gave prosecutors and the institution itself unwavering public support.