How dare he? The plummeting stock market served to divert attention from President Trump’s astonishing attack on Democrats who failed to applaud during his State of the Union address last week.

Speaking at a manufacturing plant outside Cincinnati on Monday, Trump accused Democrats of the vilest crime: treason. “They would rather see Trump do badly, okay, than our country do well,” Trump said, lamenting the “very selfish” and “bad energy” on the Democratic side of the chamber.

Even when he touted record-low black unemployment, Trump complained, Democrats sat on their hands. “They were like death. And un-American. Un-American. Somebody said ‘treasonous.’ I mean, yeah, I guess, why not? Can we call that treason? Why not? I mean they certainly didn’t seem to love our country very much.”

It’s fair to say that Democrats looked churlish in their sour response to Trump’s speech. My colleague Dana Milbank catalogued this misbehavior: the refusals to stand, the heckling, the turned back. Bad behavior; bad politics.

But not un-American. Not treasonous.

It is not treasonous, but it is un-American, actually, to accuse your political opponents of a crime against the nation for constitutionally protected dissent. It should not be necessary to spell this out, but here we are. The president does not understand the first thing about the country he was elected to lead, or about the Constitution that he swore to uphold.

For instance, that treason is the only crime defined in the Constitution, which provides that “Treason against the United States, shall consist only in levying war against them, or in adhering to their enemies, giving them aid and comfort.” Indeed, the Constitution gets hyper-technical: “No person shall be convicted of treason unless on the testimony of two witnesses to the same overt act, or on confession in open court.”

The reason for this specificity, as University of California at Davis law professor Carlton F.W. Larson has explained, is that the framers were concerned that accusations of treason not be transformed into criminal cudgels against political enemies.

“Treason,” wrote James Wilson, understood to be the author of the Treason clause, “is unquestionably a crime most dangerous to the society, and most repugnant to the first principles of the social compact. It must, however, be observed, that as the crime itself is dangerous and hostile to the state, so the imputation of it has been and may be dangerous and oppressive to the citizens.”

Dangerous and oppressive indeed. Trump surely does not imagine putting Democrats on trial for treason.  Indeed, if you watch the video, you can see him treat the outrageous accusation in an offhand, why-not-go-there manner: Treason? Whatever. But even if Trump is not ready to round up his political opponents, it is appalling — it is unthinkable — that a president would use this kind of language to describe dissent.

Nor is this the first time that Trump has gone there. In an interview with the Wall Street Journal last month he accused FBI agent Peter Strzok of treason for sending texts critical of Trump and, in Trump’s paranoid interpretation, scheming to oust him. “By the way, that’s a treasonous act,” Trump said. “What he tweeted to his lover is a treasonous act.”

That was vile enough. This is worse. We all need to stop checking our depleted retirement accounts long enough to take note of this disgraceful moment.