Republican presidential nominee Donald Trump told Hillary Clinton that if he becomes president, he will instruct the attorney general to hire a special prosecutor to investigate her. (The Washington Post)

HAS THERE been a more disturbing low in presidential politics?

“If I win, I am going to instruct my attorney general to get a special prosecutor to look into your situation,” Republican nominee Donald Trump threatened in the course of the second presidential debate Sunday night.

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

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

Mark the moment. A major-party presidential nominee is officially promising to lock up his political opponent, despite the fact that an impartial federal investigation concluded that no fair prosecutor would have charged Ms. Clinton in the matter of her emails. If anyone needed any more proof that Mr. Trump does not understand the meaning of rule of law as opposed to arbitrary rule of autocrat — that he would use the levers of the federal government in a vindictive, self-serving and corrupt manner — Mr. Trump provided it.

Republican presidential nominee Donald Trump responded Hillary Clinton saying that he shouldn't be in charge of the law in the U.S. saying, "You'd be in jail." The comment was met with rousing applause from the audience. (The Washington Post)

When the discussion turned to policy, Mr. Trump was little better. He called Syrian refugees “the great Trojan horse of all time,” as usual without evidence or foundation. He doubled down on the falsehood that he opposed the Iraq War.

Mr. Trump again defended Russian dictator Vladimir Putin, suggesting that, contrary to a finding by the U.S. intelligence community, Moscow is not attempting to intervene in the election through computer hacking. While Ms. Clinton harshly blamed Russia for aggression in Syria, Mr. Trump said he disagreed with his own running mate, Indiana Gov. Mike Pence, that the United States should act to stop bombing by Russian and Syrian forces because, Mr. Trump said, those forces are going after the Islamic State. In fact, as Ms. Clinton and Mr. Pence understand, the primary victims of Russian and Syrian attacks are civilians, especially doctors and other health-care workers.

Ms. Clinton had to account for a weekend revelation of her own — the disclosure of emails allegedly showing that she had “public” and “private” views on issues. She pointed out that she was discussing Abraham Lincoln’s efforts to pass the 13th Amendment when she made those remarks — ducking the question of whether she holds one view on, for example, trade agreements in private and another in public.

But the overwhelming impression was that Mr. Trump, with his campaign sinking and Republican office holders urging him to withdraw, decided that his best response was to snarl and double down on old falsehoods. Coming out of a disastrous first presidential debate, an even more disastrous aftermath and the release of a video in which he made disgusting comments about women, Mr. Trump dismissed his boasting of sexually predatory behavior as “locker room talk.” When that did not work, he dredged up accusations in Bill Clinton’s sordid past. It was a desperate and ineffective counterpunch.

Ms. Clinton pointed out, rightly, that Mr. Trump’s recently disclosed comments were entirely in character — just the latest in a series of insults he has issued to all sorts of vulnerable people and groups. Mr. Trump helped make her case later in the debate when, asked by a Muslim American woman about Islamophobia, the Republican nominee stoked anti-Muslim sentiment by insinuating that U.S. Muslims are hiding vital information about terrorists in the country. By contrast, Ms. Clinton said, “My vision of America is one where everyone has a place.” That is not Mr. Trump’s vision.