President Donald Trump calls him “beleaguered” and “weak” — but it’s Attorney General Jeff Sessions who actually has the power in the relationship.

White House aides have warned the president against firing Sessions, according to a former Trump administration official, because of the risk that he could be a potent weapon in special counsel Robert Mueller’s obstruction of justice investigation. And the Republican-controlled Senate has made clear it will not confirm another attorney general.

Trump has for months obsessed publicly over the perceived disloyalty of his attorney general, tweeting again this week that he wished he’d picked someone else for the job. In private, Trump has leaned on Sessions to reverse his decision to recuse himself from Mueller’s sprawling probe — and his firing now over a refusal to do so, some say, would bolster Mueller’s case that Trump has tried to block that investigation from proceeding.

The president is aware that Sessions may have the upper hand, according to two senior administration officials, and his unrelenting campaign against his attorney general is in part fueled by that knowledge.