Comparing Syrian dictator Bashar al-Assad unfavorably to Adolf Hitler, as White House Press Secretary Sean Spicer did Tuesday, was an unforced error for which Spicer was forced to apologize. Repeatedly.

“We didn’t use chemical weapons in World War II,” Spicer said. “You know, you had a — someone as despicable as Hitler who didn’t even sink to using chemical weapons.”

Spicer tried to say that using such weapons, as Assad has done, is beyond all international norms, even on the battlefield, let alone against non-combatants in a civil war. But he was tone deaf, insensitive and clumsy — not to mention historically inaccurate. For starters, even the first part of Spicer’s assertion is only true on a technicality: The U.S. didn’t use “chemical” warfare in 1945, but it ended World War II by dropping weapons of mass destruction on Nagasaki and Hiroshima, and many of those who died did so from the aftereffects of radiation poisoning.

German forces, incidentally, employed toxic gas in Eastern Crimea. The toxins used by the Nazis to kill millions of Jews in the gas chambers at Auschwitz and elsewhere were carbon monoxide and hydrogen cyanide. Again, not “chemical” weapons, strictly speaking, but who cares? And not to put too fine a point on it, but sarin gas — the nerve agent used by Assad — was invented during the war by Nazi scientists who were attempting to weaponize it when Germany fell.

Anyway, there’s a simple rule of thumb here, applicable beyond politics: If you find yourself uttering a sentence that includes the phrase “even Hitler didn’t sink to …” just stop talking and start over. Safer still, don’t talk about Hitler and domestic politics at all. Yet Hitler and Nazi references are so ubiquitous in American politics that one wonders why Spicer was hung out to dry. He isn’t even the first to make the Assad-Hitler analogy. On Sept. 1, 2013, Secretary of State John Kerry said on “Meet the Press” that Assad “now joins a list of Adolf Hitler and Saddam Hussein who’ve used these weapons in a time of war.”

So far, so good. But three days earlier, while trying to nudge President Obama to act in Syria, MSNBC’s Chris Matthews tried to make the same point — except that he was less precise than Kerry. “We didn’t use them in World War II, Hitler didn’t use them, we don’t use chemical weapons,” Matthews said.

In other words, Chris Matthews went full Sean Spicer. Why wasn’t there an outcry? Well, although Matthews was once the communications director for House Speaker Tip O’Neill, for many moons he’s been a gabfest host on cable TV — not the same thing.

The more basic problem with Hitler comparisons is that they are invariably partisan and usually absurd. No serious player in American political history can reasonably be compared with Hitler — or Stalin or Mussolini. Yet it keeps happening, whether it’s in Saratoga Springs high school curriculum, a Democratic congressman from Virginia tweeting about Trump’s “brown shirts” coming to get people, or Hollywood A-lister Ashley Judd equating Trump voters to Nazis in a foul-mouthed rant on the National Mall the day after Trump’s inauguration.

For his part, the president compared the U.S. intelligence services to “Nazi Germany” days before taking the oath of office. If it’s true, as veteran journalist Michael Kinsley wrote, that the first person to use such language has lost the argument before it begins, then both Trump and his critics are losers. “It’s ridiculous to compare any living person to Hitler or Mussolini,” Kinsley wrote. Yet even while seemingly in the realm of reason, odd impulses arise: Kinsley’s op-ed, published in The Washington Post, was titled “Donald Trump Is Actually a Fascist” — as if the modifier “actually” somehow absolved the author or the newspaper. Meanwhile, The New Republic weighed in with “Yes, Donald Trump is a fascist,” as though prefacing the argument with the word “yes” makes it valid.

And yet, at a time of disquieting discord in American politics, it is hardly out of bounds to discuss whether the conditions in our voting booths, halls of government, and streets bear a semblance to the post-World War I upheaval in Europe that led to the rise of the Axis powers and the ensuing Second World War and the genocide we know as the Holocaust. It is a fact that the fascist governments that took root in Italy and Spain and Germany featured charismatic and dangerous leaders who — at least initially — advanced at the ballot box. It is also a matter of record that in Germany the conversation wasn’t limited to political commentary and campaigns: It included pitched battles in the streets between rightists and leftists who preferred slogans and violence to reasoned argument.

But for leftists — and even thoughtful liberals — who view “The Art of the Deal” as some sort of subliminal version of “Mein Kampf,” these comparisons don’t always work to their advantage. Did Trump voters routinely shout down Hillary Clinton or go to her events and engage her supporters physically — or was it the other way around? And notwithstanding their claim to be “anti-fascists,” which side uses threats to deny the rights of minorities (in this case, conservatives) to speak and be heard and mingle freely?

Consider the example of Fresno State history professor Lars Maischak, who tweeted, “For the sake of American democracy, Trump must hang. The sooner and the higher the better.” When exposed by the conservative press, the German-born professor lashed out at his college president for not defending him unconditionally, painted himself as the victim, and whined about demands that he be deported or fired.

“To read this [his tweet] as an invitation to, or expression of intent for, murder or assassination is far-fetched,” he said. It didn’t seem that way to me, or to the Secret Service, which questioned Maischak in midweek. That seemed to do the trick: He finally apologized. But the lynch-Trump tweet wasn’t an isolated example. In other tweets, this molder of young minds called for the “execution of two Republicans for every deported immigrant,” called Trump voters “white trash scum,” and, of course, compared Trump to Hitler. He also equated Christianity with fascism, adding, “If only Mary had had an abortion!”

But just when you wanted to give up on American higher education completely, there comes along another immigrant professor, this one from another former Axis country. Italian immigrant Gianni Riotta, a journalist and Princeton professor, has seen fascism up close — and knows about the half-million lives it cost in his native country — and he says that what America is going through isn’t even close.

When he was a high school senior, Riotta’s name was put on a publicly distributed “target to be hit” list put out by a neo-fascist gang that included his phone number and home address. In other words, Riotta knows the difference between fascism and populism.

“A sense of proportion is crucial to avoid alienating voters further,” he wrote last year with a clarity that eludes so many of his fellow liberals. “Trump’s fans are too few to march on Washington, but way too many to ignore or mock. They want jobs, schools, safe communities. If you keep your eyes on their needs, and not on Trump’s distorted hall of mirrors, you will not see ‘fascists’ but instead people forgotten by both Democrats and Republicans.”

Carl M. Cannon is executive editor and Washington Bureau chief of RealClearPolitics.