Among the most consequential of Donald Trump’s Cabinet appointees so far is Jeff Sessions, the former federal prosecutor from Alabama who would take over the Department of Justice if he is confirmed as attorney general by his Senate colleagues. As Slate’s Mark Joseph Stern wrote on Friday, Sessions could use his immense power to reverse decades of advances in civil rights. His potential to influence policy on the enforcement of criminal law deserves scrutiny as well.

Sessions would take over a Justice Department that, under the leadership of Loretta Lynch and Eric Holder before her, has been oriented toward making the criminal justice system less punitive—especially toward people accused of drug crimes. Sessions has been a vocal skeptic of those efforts, defending the use of harsh mandatory minimum sentences, attacking Obama’s campaign to grant clemency to federal prisoners serving excessively long sentences, and using his influence in the Senate to help kill the once-promising legislative push to reform the federal justice system.

It’s a record that should dismay anyone who believes the United States holds too many people in prison (about 210,000 in the federal system and 1.4 million more in the states) or that the war on drugs has ruined the lives of too many people who needed help rather than punishment. To find out what Sessions would actually be able to do as attorney general to advance his severe vision of justice, I called Mona Lynch, a professor of criminology and law at the University of California–Irvine and the author of a new book called Hard Bargains: The Coercive Power of Drug Laws in Federal Court. Our conversation, which has been edited and condensed for clarity, follows below.

What was your reaction to learning Jeff Sessions had been tapped for AG?

Well, I’ll say this: When I was asked, after the election, who my nightmare person was, in terms of the drug war and the possibility of reform, Jeff Sessions was at the top of my list.

He is someone who has expressed enthusiasm for punishing criminals as harshly as possible. What would he be able to do, as attorney general, to achieve that?

He could impose very strict rules about how U.S. attorneys and their line prosecutors manage cases. Under John Ashcroft, the policy was that you had to seek convictions on the most serious provable offense. That made it so federal prosecutors were under a lot of pressure to bring the heaviest charges possible and really use drug law to its maximum force.

That said, the federal justice system is not like a military order, where policy is going to be followed down the line and any person who deviates from it is out on their butt. There will be some flexibility, but he will change the tone. My biggest concern is the return of using a sentence enhancement called the 851 in plea-bargaining on drug cases.

What is that?

Prosecutors can choose to make sentences even longer for drug defendants who have prior drug convictions. In the past, many prosecutors threatened defendants with the 851, which could double the mandatory minimum and/or bring a life sentence, if they insist on going to trial or, in some cases, if they just decline to provide substantial assistance [to prosecutors]. One of Holder’s interventions was to discourage prosecutors from using the 851 to get guilty pleas, and I could see that going away under Sessions.

There’s a belief among some criminal justice specialists that says the federal government has a limited role in determining how many people go to prison and for how long. They point out that only about 200,000 of the 1.5 million or so people who are in prison are in the federal system, while the rest are prosecuted at the state level. What can Sessions do with his power to undermine that view?

They are right that most criminal justice activity is at the state and local levels. And at this moment in time, I’m really rooting for bottom-up reform, efforts to double down and really protect people. That said, there are a couple of places where that’s not going to matter, and the biggest one, of course, is immigration. That is a core federal area of criminal law that states are not involved in, and it intersects with the drug prosecutions on the border.

Also, the federal government has a lot of jurisdictional power on drug cases—it has the ability to take a low-level state case and elevate it to a federal court and seek a much longer sentence. There’s no need for there to be any crossing of state borders—just a regular old deal on the street can end up in federal court. That has an effect on how local cases get dealt with, because it creates the threat of, “We can go federal on you if you don’t agree to this deal or provide us with information.”

The third thing that I worry will happen is that it’ll move away from the white-collar stuff that was pursued under Holder and Lynch. It’s going to go back to this much more traditional tough-on-crime model.

What influence does the AG have over how the U.S. attorneys pick cases and what charges they bring?

My guess is Sessions will be advising on appointments of U.S. attorneys at the district level. There will be some turnover in those offices, which will then trickle down into how policy gets implemented by line prosecutors. All of them work under the Department of Justice, with the AG at the top. That said, it’s not like Sessions will be going into every district, hiring and firing employees based on their behavior. But he will set policy and send directives and set the standard operating procedure for U.S. attorneys’ offices.

What does setting policy mean in this context?

The AG may develop standards of conduct or issue memoranda to the U.S. attorneys’ offices saying, “This is how you’re supposed to handle cases.” So for instance, he could say—as John Ashcroft did—“You must seek approval from Justice to make a plea bargain for something less than the most serious provable offense.” What that is saying, in essence, is you can’t give people sentencing breaks by plea-bargaining away provable elements of a crime.

All such directives get laid out in the U.S. attorneys’ policy manual, which gets updated periodically. In my book, I trace how that document changed with the war on drugs and how the rhetoric changed from “leave small drug stuff to the states” to “it’s our responsibility to go after drug dealers.”

How do you think Sessions would change the policy manual, besides making it a requirement that prosecutors charge people with the highest provable offenses?

My other fear is he’ll reverse Eric Holder’s first big move, from 2013, where he said, “Don’t charge the drug weight if it’s going to trigger a mandatory minimum for a low-level defendant.” My guess is that will go away.

That was a really consequential move, in that it resulted in a huge drop in the use of mandatory minimums in drug cases. But it worked because prosecutors around the country believed it was a good guideline, right? They didn’t have to comply with it? If Sessions becomes AG and issues guidelines that make the system more punitive, what kind of room will prosecutors have to say, “Thanks for the advice, but we’re not going to go along with it”?

Well, I think we’re going to see some changes in personnel. They’re going to get people who are aligned with them and who will not buck the directives. But part of what this is likely to do is deepen and solidify the very big differences that already exist among districts. So, places that have begrudgingly gone along with the move away from the drug war, they’ll be able to jump right back in. Sessions was a U.S. attorney in the southern district of Alabama—they may say, “We here in Alabama really want to go after the drug dealers. We want this drug war. We feel like that’s what our job is, and we’re going to go full force now.”

Do you have any predictions about how Sessions will adapt the old drug war philosophy to the opioid crisis?

I think he’s made it clear. He gave a statement, I think it was in March, about the opioid crisis, and basically he sees it as being all about Mexico and prosecuting drug dealers. So he’s making that exact bifurcation that you can’t make, which is, there are people called “addicts” who suffer, and then there are people who “peddle” drugs, in his words, who are evil thugs who have to be handled with a hammer. So he’s making a bifurcation that completely ignores the complexities of the drug problem. You can’t just say, “We can treat these addicts” and “We can hammer these dealers.” We know what’s going to happen. Prices will go up. People won’t stop using drugs. There will be more collateral consequences from higher prices, perhaps more violence in the drug markets. It’s just kind of a very simplistic, old-fashioned approach to dealing with the drug problem.

What aspects of Sessions’ career as a federal prosecutor make you worried about what kind of AG he’d be?

Well, he has been a vocal proponent of using snitch evidence, or confidential informant evidence, against people. And he has not publicly expressed an awareness of the real troubles that come along with that. He has a kind of confidence in the face of the very real reliability problems that we know exist with that kind of testimony. When he was a U.S. attorney, his office was known for using uncorroborated snitch evidence against people to get very, very long sentences and also using bigger dealers to nail small-time people.

What is it about his worldview that causes him to go to the most extreme direction on all these fronts? Why do you think he is willing to overlook the dangers of using confidential informants and ignore the blurry lines between addicts and dealers?

That’s a tough one. To me, that kind of certainty—that black-and-white worldview, where there’s good and there’s evil and there’s no gray in the world—is very scary. But it is definitely a type of leadership—it’s an ability to move forward and do things that are extreme without questioning them. So it’s a mode of thinking that actually allows you to get things done that would trouble other people. That kind of confidence is, I think, a powerful and scary trait, because it allows you to reinterpret things to fit with what you want done and to ignore contrary evidence.