Mr. Sessions has made no decision, and in soliciting the assessment of department lawyers, he may be seeking a way out of the bind his boss has put him in by effectively putting the matter in the hands of professionals who were not politically appointed. But if he or his deputy authorizes a new investigation of Mrs. Clinton, it would shatter norms established after Watergate that are intended to prevent presidents from using law enforcement agencies against political rivals.

The request alone was enough to trigger a political backlash, as critics of Mr. Trump quickly decried what they called “banana republic” politics of retribution, akin to autocratic backwater nations where election losers are jailed by winners. The issue will almost certainly energize what was already shaping up to be a contentious hearing scheduled for Tuesday morning, when Mr. Sessions is scheduled to testify before the House Judiciary Committee.