From The New York Times:

“Just because you can do something doesn’t mean you won’t get impeached for doing it,” said Cameron Smith, a former aide to Mr. Sessions. “What’s happening here is you have a president saying, ‘I can do what I want to do.’ Well, are you going to do it or not? The answer is no because he does not have the political cover.”

[…]

But Mr. Smith, the former aide to Mr. Sessions who is now a vice president at the R Street Institute, a free-market-oriented research organization, said the president had also shied away from firing the attorney general because Mr. Sessions is popular with the same conservative supporters that Mr. Trump needs to survive his many travails.

“What we’ve got here is a president trying to damage Sessions so he can remove him without political damage,” Mr. Smith said. “He still needs the political base.”