President Barack Obama pauses as he speaks at the El Reno Federal Correctional Institution in El Reno, Okla., Thursday, July 16, 2015. AP Donald Trump has not said much about how he would handle matters of criminal justice if he is elected president.

Beyond a promise in his speech at last month’s Republican National Convention that “safety will be restored” in America — and a suggestion in December that he would seek the death penalty for anyone convicted of killing a police officer — the candidate has not articulated a policy agenda on issues such as the drug war, federal sentencing guidelines, community policing or clemency.

Yet Trump has made it clear what he will undo: the Obama administration’s executive actions and regulations, including those having to do with criminal justice.

“You know, the great thing about executive orders is that I don’t have to go back to Congress,” he said at a campaign rally in Manassas, Va., on Dec. 2, according to the Daily Caller.

Experts on executive authority say the next president could absolutely — and immediately — rescind any and all executive orders made by President Obama during his eight years in office, including those tighteningbackground checks, “banning the box” on federal job applications and banning the solitary confinement of juveniles in federal prisons.

“They can be overturned in one day, with the stroke of a pen,” said Susan Dudley, a professor of public policy at George Washington University and an expert on regulatory procedure.

Trump could also opt to slow-walk Obama’s policies — either by appointing cabinet officials who will not enforce them or by instructing his Justice Department to reprioritize which laws it will prosecute. The department could also reach weakened, out-of-court settlements in the investigations that the Obama administration has launched into local police departments.

But other moves of Obama’s, from his pardons and commutations to his attempts to ease sentencing guidelines for drug offenders, will be harder to roll back. “Trump could say he doesn’t want to pursue a certain policy anymore, but he can’t take away benefits and rights that have already gone out to people,” said Stephen Vladeck, a law professor at the University of Texas and an expert on constitutional law and the federal system.

Below, a rundown of Obama’s efforts on criminal justice and how each of them could or could not be unraveled by a President Donald Trump.

Gun control

In early January, President Obama announced a set of “executive actions” on guns.

For example, the president moved to close the “gun-trust loophole,” through which felons, domestic abusers, and people with mental illnesses had been able to purchase assault weapons by registering them to a corporation, thereby avoiding a background check.

Obama also expanded the background check system itself, by hiring more than 200 FBI agents and offering incentives to states for reporting information on gun buyers. And he clarified who must be licensed as a “gun dealer,” among several other reforms.

Before Obama officially took these actions, Trump had promised to undo them: “I will veto. I will un-sign that so fast,” he said at a rally in Biloxi, Miss.

Such a reversal may be easier said than done. A Trump administration could surely re-redefine what a “gun dealer” is, re-assign FBI agents away from the task of performing background checks, and roll back the current Justice Department’s efforts to track lost, stolen, and crime-involved guns.

But he could not withdraw the training and resources that the Obama administration has already provided to schools around the country to prepare them for shootings. Nor could he undo research that the administration has already done into “smart gun” technology.

And if he wanted to reopen the gun-trust loophole, his administration would need to invest perhaps its whole first term in doing so.

“There’s a process for undoing a regulation,” said Dudley, the regulatory expert. “But it’s complicated and cumbersome. They have to rewrite the regulation, have a legal justification for it, wait for public comments, evaluate the public comments... It could take years.”

Clemency

In 2014, the Obama administration unveiled a Justice Department program to offer executive relief to federal prisoners serving long sentences for nonviolent drug crimes. The initiative has seen its share of problems, but Obama has now used commutations to shorten the prison sentences of 562 people, more than the previous nine presidents combined.

A President Trump could not reverse these commutations. “That would be a due process issue,” said Vladeck, the constitutional law expert. “No way he could do that.”

However, Trump could certainly stop issuing pardons and commutations at the rate that Obama has, and his attorney general could put a stop to the clemency initiative, leaving behind the almost 2,000 prisoners who qualify for relief under the program’s criteria but whose cases have not been decided.

Sentencing

Under another initiative, former Attorney General Eric Holder instructed federal prosecutors across the country to charge fewer people with low-level, nonviolent drug crimes and instead focus on more serious, violent crime. In the first year after the program was implemented, the number of drug offenders charged under mandatory-minimum sentencing guidelines fell by 20 percent.

A President Trump could quickly re-set these priorities, by having his new attorney general instruct prosecutors to start bringing those harsher drug charges again.

“A substantial part of that progress would be at risk if Trump were to be elected,” Holder recently told The New York Times. “I don’t think there’s any question.”

“Banning the Box”

Speaking in Newark, N.J., last November, Obama announced a host of executive actions designed to help ex-offenders reenter society. These included grants to legal aid organizations and to jobs, housing, and reentry programs across the country, among several other moves.

Most prominently, the president moved to “ban the box” on federal job applications, prohibiting government agencies from preemptively asking prospective employees about their criminal history.

As soon as Obama issued that order, Sen. Cory Booker of New Jersey warnedthat it “should be done legislatively, so that the stroke of another president’s pen can’t undo it.”

Indeed, Trump could reinstate “the box” — but he would need to go through the same lengthy process with the Office of Personnel Management that the Obama administration did.

Solitary confinement

Within a few months, Obama issued another set of executive actions, this one limiting the use of solitary confinement in the federal prison system. He called for a prohibition on using solitary as a punishment for “low-level” infractions; a cap on the number of consecutive days (and hours) a prisoner can spend in solitary; alternatives to solitary for the mentally-ill; and the hiring of more psychologists.

Obama also banned outright the solitary confinement of juveniles in federal prisons.

Trump could rescind all of this right away. After the Bureau of Prisons adjusts to his administration’s new rules, a small number of juveniles could find themselves back in solitary again.

Militarization of police

Last May, Obama announced that he was prohibiting federal agencies from transferring certain military-style equipment — including armored vehicles, grenade launchers, and ammunition of .50 caliber or higher — to local police departments, at least until those requesting the gear underwent federal training.

Police in Ferguson, Missouri, far beyond your friendly neighborhood patrolmen. Larry W Smith/EPA

Trump could remove this ban, reopening the flow of such weapons to local police.

Higher education for prisoners

And last July, the Obama administration announced a program to enroll roughly 12,000 incarcerated students in college programs, paid through federal Pell grants. A provision of the now-controversial 1994 crime bill had prohibited all prisoners from receiving these grants, but Obama circumvented that rule by calling the new initiative a “pilot” program only.

Trump could shut down this program, though it would not be possible to retract the education that hundreds of prisoners will have already received.