In Sunday night’s presidential debate, Donald J. Trump told Hillary R. Clinton, “if I win, I am going to instruct my attorney general to get a special prosecutor to look into your situation.”

When Ms. Clinton responded, “it’s just awfully good that someone with the temperament of Donald Trump is not in charge of the law in our country,” Mr. Trump intoned, “Because you would be in jail.”

It was a breathtaking moment in American history that a candidate for president would threaten to jail his opponent. This is exactly the kind of vengeance against political enemies that led the colonists to break free from Britain in 1776 and that today we associate with authoritarian regimes in less developed parts of the world.

As a federal prosecutor for 17 years, during both Democratic and Republican administrations, I never heard anything approaching Mr. Trump’s assault on the nonpartisan principle that there is no room for politics in criminal prosecutions.

In August 2000, I was the newly appointed chief of the Justice Department’s environmental crimes section, and a federal grand jury was investigating Koch Industries and Koch Petroleum Group for possible criminal violations of the Clean Air Act. As Republicans gathered in Philadelphia to nominate Texas Gov. George W. Bush, rumors swirled that the Clinton Justice Department would indict the Koch companies that week to embarrass Governor Bush.

At the Justice Department, we reacted to the news reports with bewilderment. We had no idea who the Koch brothers were or that they would become such pivotal figures in American politics. But even if we had known, it would have made no difference. Politics should never influence decisions about prosecutions.

Federal prosecutors have enormous power, none greater than their unfettered ability to conduct criminal investigations and to pursue criminal charges. The decision to prosecute can destroy lives, even when charges are later dismissed or when a jury acquits the defendant. Even the existence of a criminal investigation and the threat of criminal prosecution can irreparably damage reputations.

For that reason, there are few principles more sacred to the rule of law than the understanding that we should never politicize criminal prosecution and never deploy criminal charges as a political weapon. It is shocking that former New York Mayor Rudy Giuliani and New Jersey Gov. Chris Christie, both of whom were United States attorneys, have argued as surrogates for Mr. Trump that Mrs. Clinton should be prosecuted. As former prosecutors, they know better.

I served under both Democratic and Republican presidents, all of whom assiduously stood by the principle that law enforcement should never be used for political purposes. In a time-honored approach, the White House is not allowed to contact prosecutors about cases they are handling, and neither is the Congress.

Prosecutors must make their decisions based solely on the law and the facts, tempered only by their obligation to do justice for our citizens.

No president should ever use the Justice Department to attack political opponents. No principle is more fundamental to how we ensure the integrity of our democracy.

The New York Times

David M. Uhlmann, a law professor at the University of Michigan, was chief of the environmental crimes section at the Justice Department from 2000 to 2007.