It was an otherwise unremarkable event when Donald Trump received the endorsement of the New England Police Benevolent Association, a union of police and correctional officers in Massachusetts and New Hampshire, on December 10. As he addressed the crowd in Portsmouth, New Hampshire, Trump restated his support for police officers and for the death penalty for those who kill them. Then he articulated a new proposal to demonstrate that support.

One of the first things I’d do in terms of executive order if I win would be to sign a strong, strong statement that will go out to the country, out to the world, that anybody caught killing a policeman, policewoman, police officer, anybody killing a police officer: death penalty. It’s gonna happen. OK? We can’t let this go.

At least three problems with this idea spring to mind.

First, the U.S. Supreme Court forbade mandatory death sentences in 1976 with its ruling in Woodson v. North Carolina. Central to the Court’s ruling was the justices’ opposition to punishing all murderers alike without regard for the aggravating or mitigating circumstances of each case. But the justices also feared that mandatory death sentences would compel jurors to hand down not-guilty verdicts for otherwise guilty defendants who they did not think deserved to die. Although Justice Clarence Thomas hinted at the possibility of revisiting Woodson in his Glossip v. Gross concurrence in June, the other justices did not seem eager to do so.

Second, the death penalty is largely administered by the states, not the federal government. Roughly 3,000 inmates currently sit on death row in the United States; only about 60 of them are in the federal system. President Trump would have no lawful power to influence state criminal-justice systems, whether by executive order or any other mechanism at his disposal. Any efforts to the contrary would violate the federal character of the Constitution.

Finally, and most importantly, the president doesn’t have the lawful power to unilaterally impose a criminal punishment on anyone, whether it be a fine, a prison sentence, or death. Presidents can wield the pardoning power to reduce or remove punishments for federal crimes, but they can neither increase nor enact them. The American legal system delegates that responsibility to judges and juries. Infringing on that separation of power through executive order would, at minimum, violate the Fifth and Fourteenth Amendments’ guarantees of due process.

Trump has a history of invoking the death penalty without regard for its limitations. After the brutal rape of a white jogger in Central Park in May 1989 received widespread media attention, and amid a rise in crime rates nationwide, Trump took out a full-age ad in four New York City newspapers with the title “BRING BACK THE DEATH PENALTY! BRING BACK THE POLICE!” He did not specifically reference the Central Park jogger attack in the ad, but its timing made the connection inescapable.

Mayor [Ed] Koch has stated that hate and rancor should be removed from our hearts. I do not think so. I want to hate these muggers and murderers. They should be forced to suffer and, when they kill, should be executed for their crimes. They must serve as examples so that others will think long and hard before committing a crime or an act of violence. Yes, Mayor Koch, I want to hate these murderers and I always will. I am not looking to psychoanalyze them or understand them, I am looking to punish them. If the punishment is strong, the attacks on innocent people will stop. I recently watched a newscast trying to explain “the anger in these young men.” I no longer want to understand their anger. I want them to understand our anger. I want them to be afraid.

How can our great society tolerate the continued brutalization of its citizens by crazed misfits? Criminals must be told that their CIVIL LIBERTIES END WHEN AN ATTACK ON OUR SAFETY BEGINS!

But Trump’s desire to make them “understand our anger” through the death penalty could not be lawfully sated. Even if New York had allowed capital punishment at the time—it eventually returned in 1995—and even if Manhattan District Attorney Robert Morgenthau—a staunch opponent of the death penalty—had wanted to seek it, none of the perpetrators could have received it.

When Trump published his full-page ads, police had already arrested five suspects for the crimes, all of whom were young black and Hispanic men between the ages of 14 and 16. Each had been named in connection with unrelated beatings and attacks in the park that night. Of the five teenagers, who would later be known as the Central Park Five, four were 14 or 15 years old. The U.S. Supreme Court had ruled the previous year in Thompson v. Oklahoma that executing a 15-year-old would be cruel and unusual punishment. However, the fifth defendant had been 16 years old at the time of the attack, and the Court had upheld the death penalty for 16- and 17-year-olds in Stanford v. Kentucky in the summer between his arrest and his trial.

But even if all of the Central Park Five had been old enough to qualify for death sentences, none of them could have been executed for the crime. The Supreme Court had already abolished the death penalty for rape over a decade earlier in the 1977 case Coker v. Georgia. The Court’s opinion, written by Justice Thurgood Marshall, avoided citing the vast racial disparities in death sentences for rape in its reasoning. But the justices, especially Marshall, were aware of those disparities and likely motivated by them. Between 1930 and 1972, only Southern and border states still imposed the death penalty for rape; over 90 percent of those executed for it were black.

All five teenagers were eventually tried, convicted, and sentenced to multiple years in prison for the Central Park rape. Then, years later, it was revealed that none of them had actually committed the crime.

The “park marauders,” the “roving gang,” the “crazed misfits” were fourteen, fifteen, and sixteen years old. The confessions they gave, as children, had been false, spun out under the pressure of hours of police interrogations. (They were, had anyone been ready to acknowledge it at the time, also inconsistent; they also had parents whom they weren’t able to see before their questioning.) The boys were sent to prison. One of them, Kharey Wise, who at sixteen was the oldest and sentenced as an adult, was still there when, eleven years after the rape in the park, he happened to cross paths with a prisoner named Matias Reyes. It occurred to Reyes that it was his fault that Wise was there. He confessed that he, and he alone, had raped and beaten Meili, as he had raped other women over the years. He described to police how he had tied her with her clothes; it had been part of his M.O. in other cases, something that gave credibility to his confession. It moved beyond a doubt when a DNA test matched Reyes to the semen found on Meili’s body. The DNA hadn’t matched any of the teen-agers—one of the many details that got blinked over in the trial. They were exonerated twelve years ago, and the charges were formally dropped.

The Central Park Five sued the city for their wrongful prosecution and received a $40 million settlement in 2014, $1 million for every year of their lives wrongfully spent behind bars. Shortly after the news of the settlement broke, Trump published an op-ed in the New York Daily News calling it “a disgrace.”

Forty million dollars is a lot of money for the taxpayers of New York to pay when we are already the highest taxed city and state in the country. The recipients must be laughing out loud at the stupidity of the city.

Speak to the detectives on the case and try listening to the facts. These young men do not exactly have the pasts of angels.

Trump was far from the only New Yorker who rushed to condemn the Central Park Five in the heated summer of 1989. And, as a real-estate businessman and private citizen, he was free to propose any number of unconstitutional solutions to complex social issues.

But on the campaign trail, presidential candidates make proposals with the implication that those policies can actually be carried out. Political reality might intrude on pledges like Obama's first-term promise to close Guantanamo Bay. But that pledge itself doesn’t violate the Constitution. Ordering executions by executive decree is a campaign promise Trump can’t lawfully fulfill.