Victoria Bassetti is the author of “Electoral Dysfunction” and a contributor at the Project on Government Oversight and the Brennan Center for Justice. Caroline Fredrickson is president of the American Constitution Society and the author of “Under the Bus.”

The Justice Department under Jeff Sessions, if he is confirmed as President Trump’s attorney general, will be free to prioritize marijuana prosecutions over civil rights investigations of police departments, or to focus more on voting fraud than environmental crimes. Elections have consequences. Changed law enforcement priorities are among them. That’s politics.

It is not, however, “politicization” of the Department of Justice. That’s an altogether different — and more dangerous — phenomenon. It’s what happens when an attorney general or president employs the enormous power of the department, with its 10,000 lawyers and 13,000 FBI agents, to pursue personal or partisan goals. It happens when impartiality is thrown out the window and vindictiveness and vendetta take over.

(Jenny Starrs/The Washington Post)

Is Sessions the man to ensure this does not happen? This week’s showdown between President Trump and acting attorney general Sally Yates changes the equation. The independence of the department is under threat. Protecting it must be Sessions’s top priority. Yet thus far he has offered little more than bland assurances of fairness — combined with worrying hints of blindness to the gravity of the situation.

Monday’s firing of Yates represents an ominous signal that Trump does not understand the department’s crucial role in impartially administering justice. Rather than pausing and listening to a well-respected career prosecutor’s profound concerns, the president responded with summary termination and a statement that she had “betrayed the department.”

If this were a singular event — just a high-stakes confrontation with an Obama administration holdover involving a controversial presidential decision — it might not be so alarming. But there are other signs of looming politicization, and of Trump’s failure to understand the need for clear lines of division between the Justice Department and his White House.

While on the campaign trail last fall, Trump threatened to kill AT&T’s proposed merger with Time Warner — “a deal we will not approve in my administration.” Then in January, Trump blasted Time Warner’s CNN (“Your organization is terrible”) at his first news conference as president-elect. So AT&T’s chief executive made a pilgrimage to Trump Tower to try to mollify the president, and worryingly, we don’t know what was said or promised at the meeting. Never mind that the proposed merger is a law-enforcement matter for the Justice Department; Trump’s behavior hints that he feels merger decisions should be based at least in part on his snap judgment or personal animus toward a company or business sector.

We’ve seen this movie before. In 1971, President Richard Nixon called Deputy Attorney General Richard Kleindienst about the department’s handling of an antitrust case involving International Telephone and Telegraph (ITT). “The ITT thing — stay the hell out of it. Is that clear?” Nixon instructed. “My order is to drop the Goddamn thing. Is that clear?”

It does not strain the imagination to envision Trump making a call like that. Not too long ago, he repeatedly threatened to jail Hillary Clinton. After the election, he reversed position. Both statements suggest he thinks that the decision whether to prosecute should be made based on his whims.

Trump has many other reasons to want to meddle with the department. For example, the FBI is investigating Russian interference with the election. In addition, the department is overseeing a $7.2 billion settlement with Deutsche Bank, which holds more than $300 million of Trump’s debt. Other possible conflicts abound.

(Peter Stevenson/The Washington Post)

Possibly aware of concerns about potential interference, Sessions said the right things at his nomination hearing. The attorney general “must be willing to tell the president or other top officials ‘no’ if he or they overreach,” Sessions said. “He or she cannot be a mere rubber stamp to any idea the president has.”

These are the right sentiments. They are also the sentiments that got Yates fired this week. But is Sessions prepared to go that far? Will he really say no to the president?

Sessions’s record of close personal and political relationship with Trump suggests perhaps not. He was a featured speaker at nine Trump rallies during the campaign. He officially nominated Trump at the Republican National Convention in Cleveland.

Yet despite his role as frequent surrogate for the president during the election, he said he would not recuse himself from investigations into the 2016 election.

There’s no magic “I will be fair” wand to cure this dilemma. Sessions must provide a more detailed explanation of how he will insulate the department and himself from improper interference, and the White House counsel’s office should make clear its rules for avoiding interference with the administration of justice. This week’s confrontation with Yates shows how high the stakes are.