By international and historical standards, political violence is exceedingly rare in the United States. The last serious outburst was 1968 with its bloody Democratic-convention riots. By that standard, 2016 is, as yet, tame. It may not remain so.

The political thuggery that shut down a Donald Trump rally in Chicago last week may just be a harbinger. It would be nice, therefore, if we could think straight about cause and effect.

The immediate conventional wisdom was to blame the disturbance on the “toxic climate” created by Trump. Nonsense. This was an act of deliberate sabotage created by a totalitarian left that specializes in the intimidation and silencing of political opponents.

Its pedigree goes back to early 20th-century fascism and communism. Its more recent incarnation has been developed on college campuses, where for years leftists have been taunting, disrupting and ultimately shutting down and shutting out conservative speakers of every stripe — long before Donald Trump.

The Chicago shutdown was a planned attack on free speech and free assembly. Hence the exultant chant of the protesters upon the announcement of the rally’s cancellation: “We stopped Trump.” It had all of the spontaneity of a beer-hall putsch.

Given the people, the money and the groups (including MoveOn.org) behind Chicago, it is likely to be replicated, constituting a serious threat to a civilized politics. But there’s a second, quite separate form of thuggery threatening the 2016 campaign — a leading candidate who, with a wink and a nod (and sometimes less subtlety), is stoking anger and encouraging violence.

This must be distinguished from what happened in Chicago, where Trump was the victim and for which he is not responsible. But he is responsible for saying of a protester at his rally in Las Vegas that “I love the old days. You know what they used to do to guys like that … ? They’d be carried out on a stretcher, folks.”

He told another rally that if they see any protesters preparing to throw a tomato, to “knock the crap out of them … I promise you I will pay for the legal fees.” Referring in an interview to yet another protester, Trump said “maybe he should have been roughed up.”

At the Vegas event, Trump had said, “I’d like to punch him in the face.” Well, in Fayetteville, N.C., one of his supporters did exactly that for him — sucker-punching in the face a protester being led away. The attacker is being charged with assault.

Trump is not responsible for the assault. But he is responsible for refusing to condemn it. Asked about it, he dodged and weaved, searching for extenuation. “The man got carried away.” So what? If people who get carried away are allowed to sucker-punch others, we’d be living in a jungle.

Trump said that it was obvious that the cold-cocker “obviously loves his country.” What is it about punching a demonstrator in the face that makes evident one’s patriotism? Particularly when the attacker said on television, “Next time we see him, we might have to kill him.”

Whoa! That’s lynch talk. And rather than condemn that man, Trump said he would be instructing his people to look into paying his legal fees.

This from the leader of the now strongest faction in the Republican Party, the man most likely to be the GOP nominee for president. And who, when asked on Wednesday about the possibility of being denied the nomination at the convention if he’s way ahead in delegates but just short of a majority, said: “I think you’d have riots,” adding “I wouldn’t lead it but I think bad things would happen.”

Is that incitement to riot? Legally, no. But you’d have to be a fool to miss the underlying implication.

There’s an air of division in the country. Fine. It’s happened often in our history. Indeed, the whole point of politics is to identify, highlight, argue and ultimately adjudicate and accommodate such divisions. Politics is the civilized substitute for settling things the old-fashioned way — laying your opponent out on a stretcher.

What is so disturbing today is that suffusing our politics is not just an air of division but an air of menace. It’s being fueled on both sides: one side through organized anti-free-speech agitation using Bolshevik tactics; the other side by verbal encouragement and threats of varying degrees of subtlety.

They may feed off each other but they are of independent origin. And both are repugnant, both dangerous and both deserving of the most unreserved condemnation.

© 2016, The Washington Post Writers Group