"Conservatives think liberals are stupid. Liberals think conservatives are evil."

Like many generalizations, this "fundamental law" of American politics, as outlined by conservative commentator Charles Krauthammer some 15 years ago, is overly broad. But it nonetheless captures something important and true about our world. Let's focus on the latter half of this maxim. It's true that liberal writers, journalists, and policy intellectuals have long expressed a level of moral outrage and even disgust about their ideological opponents that rivals and often surpasses what one typically encounters on the other side. (At the grassroots, the reverse has tended to be true, with conservatives often directing greater animosity at their ideological opponents.)

As President Trump has moved the Republican Party away from conservatism and in the direction of right-wing populism, nationalism, and anti-globalism, the liberal tendency toward moral denunciation hasn't diminished. On the contrary, it's only intensified, leading progressives to double down on their longstanding habit of seeking wherever possible to excommunicate the right from the realm of democratic argument and debate.

If liberals hope to regain the ground they've lost in recent years, they really need to change these tactics, which as often as not are self-defeating.

As I've argued on previous occasions, declaring opponents unacceptable, illegitimate, and out of bounds is a perennial temptation. That's because politics always takes place on two distinct levels. On one level is the back and forth of partisan conflict, involving persuasion, argument, electoral battles, triumphs, and defeats. On this level, pretty much anything goes as long as it abides by the rules of the political game. But there's also a second, more fundamental level of politics that involves a competition over who gets to set those rules, the boundaries of what is publicly acceptable — and precisely where those boundaries will be positioned.

Far more than conservatives, liberals love to rule certain positions out of bounds in this second-order sense. They do this by appealing to the courts — the branch of government that reviews, alters, and overturns the rules of the political game. They also do it in the important institutions they control within civil society — such as mainstream media outlets, universities, corporations, movie studios, and other arms of the entertainment industry. When these institutions informally decide that an issue, or a specific position on an issue, is simply unacceptable because it crosses a moral line that leading members of these institutions consider inviolable, they render it beyond the pale. As I wrote in a previous column on the subject, "Over the past several decades, a range of positions on immigration, crime, gender, and the costs and benefits of some forms of diversity have been relegated to the categories of 'racism,' 'sexism,' 'homophobia,' 'white supremacy,' or 'white nationalism,' and therefore excluded from first-order political debate."

Trump's presidential campaign succeeded in part because the candidate challenged these second-order taboos (especially as they show themselves in the phenomenon of political correctness) — and liberals have responded in part by attempting to reinforce the taboos, mostly through name-calling that boils down to the assertion, "You can't say that!"

Sometimes this assertion is merely rhetorical. But at other times, in the statements of various courts that have blocked Trump's policies on immigration and sanctuary cities, it's backed up by the force of the judiciary. (In France, Marine Le Pen faces a similar dynamic, with nearly the entirety of the French political establishment closing ranks against her to convey the message to the electorate that voting for the National Front is simply unacceptable.)

The problem with telling people that they're not allowed to get their way on certain issues is two-fold. First, as we've seen with the Trump phenomenon, controversial opinions don't just disappear when members of the establishment rule them out of bounds. They often reassert themselves later, more powerful and more radicalized than before. And second, the excommunicators may become fond of the tactic and apply it to an ever-expanding range of issues.

For a vivid recent example of what can happen to political thinking and debate when one side becomes wedded to upholding rigid and exceedingly narrow strictures on permissible opinion, take a look at the blistering (and bizarrely disproportionate) reaction of liberals to Bret Stephens' debut column in The New York Times. Now, I was no fan of Stephens' writing in The Wall Street Journal, where he recently resigned, especially when it came to foreign policy. Neither did I appreciate his stance on environmental issues, which struck me as overly dismissive of evidence for climate change.

But in his first Times column, Stephens came right out and described global warming, along with evidence of "human influence on that warming," as "indisputable." That sounded unobjectionable to me — as did his overarching point, which was that those who favor policies to combat climate change would convince more people to go along if they sounded somewhat less absolutely, positively, unwaveringly, indisputably certain in their predictions about what is always, after all, an all-too-uncertain future.

Stephens himself predicted in the column that his humble case for humility would cause heads to explode, and sure enough they did. Liberals on Twitter sputtered in indignation, as did several center-left news sites. The Times had hired an apologist for climate change "denialism," proclaimed Slate. According to Vox, he was a "climate change bullsh--ter." (The Week, too, was not immune.) No wonder climate scientists and many others lined up to cancel their subscriptions to the newspaper in protest.

Except that none of it was true. Stephens didn't deny the reality of climate change. He merely dared to advocate a slight rhetorical adjustment to the way environmental activists and their cheering sections at websites like Slate and Vox, and newspapers like the Times, go about making their case to the wider public. What followed was not a reasoned debate about the rhetorical effectiveness of claims to modesty and certainty, dispassionate concern and outright alarmism. Instead, there was simple, pure, satisfying, but politically impotent condemnation: "You can't say that!"

But of course he can. And he will.

Which means the all-important question for liberals remains: What then?