About 20 minutes into the debate, Donald Trump delivered a menacing threat to Hillary Clinton. “If I win,” he warned, “I’m going to instruct my attorney general to get a special prosecutor to look into your situation, because there’s never been so many lies, so much deception.”

Mr. Trump’s promising on national television to use the power of the president’s office to prosecute his chief political rival, to her face, was chilling enough.

But when Mrs. Clinton responded, Mr. Trump dropped the threat of an official investigation and any veneer of the rule of law.

“It’s just awfully good that someone with the temperament of Donald Trump is not in charge of the law in our country,” Mrs. Clinton observed.

“Because,” Mr. Trump replied “you’d be in jail.”

It’s hard to think of anything Mr. Trump could have said to more powerfully underscore the truth of Mrs. Clinton’s point. He said, in a widely watched televised presidential debate, that if he became president, he would put political opponents in cages. That’s dictator talk. But it’s not Mr. Trump’s open contempt for the norms of liberal democracy that made my blood run cold. It was the applause that came after. It is the fact that it’s no longer assured that you automatically lose a presidential debate in which you promise to jail your political rival.

Op-ed by Will Wilkinson; originally run in The New York Times