Donald Trump loves to denounce those effete liberals who are overly focused on being "politically correct."

In some ways, this isn't new. The term "politically correct" has been around for a long time. As a young conservative in the early 1990s, I remember perusing the ads for witty anti-PC T-shirts at the back of National Review. My favorite listed two columns of terms: "Real" words and politically correct alternatives: "Fat" contrasted with "Gravitationally Challenged"; "Stupid" contrasted with "Intellectually Hampered." And so on.

But never before has the crusade against political correctness been the centerpiece of a presidential campaign.

Aside from his attacks on immigration, trade deals, and the Obama administration's handling of Islamic terrorism, no topic comes up more often or more consistently in Trump's public remarks and in his tweets. Trump wants voters to know that he is unafraid of offending people if that's what's required to keep Americans safe and to confront the country's biggest problems.

In taking this stance, Trump is responding to (and undoubtedly intensifying) real and widespread concerns. Recent polls show that a majority of all Americans (and even higher numbers of Republicans) consider political correctness to be a "big problem." Yet exactly what the term even means and precisely what these many millions of Americans are objecting to is surprisingly difficult to determine — as is devising a suitable response, at least for liberals who favor something approaching absolute freedom of speech, thought, and expression.

On college campuses, the obsession with being PC is a real thing — and sometimes a real problem, especially when university administrators impose rules regulating speech and back them up with the threat of sanction. Such rules would be acceptable in a church and in many other private enterprises. But a university exists for the purpose of encouraging free inquiry into the truth. The idea of censoring speech or thought in such an environment is deeply troubling for many, and for good reason.

Though right-wing radio personalities and websites always love a good story of censorship on campus, that isn't primarily what Trump and his supporters mean when they rail against political correctness. They mean something subtler: not formal rules in private institutions (let alone legal strictures in the nation's public life, where the First Amendment remains very much in effect) but rather informal moral sentiment in the country at large. The anti-PC brigade bristles at the suggestion that some terms and opinions have been deemed unacceptable and ruled out of bounds by various authorities and gatekeepers in civil society, above all by the incorrigibly liberal mainstream media and government bureaucrats.

But of course Trump and his supporters aren't defending an easygoing, open-ended relativism in public speech and expression as an alternative. On the contrary, as we've seen with the vociferous reaction to NFL quarterback Colin Kaepernick's decision to protest racism by sitting out the national anthem before San Francisco 49ers games, the right is quite willing to unite in denouncing certain forms of incorrect speech and expression.

What the right doesn't like is having its own opinions on a range of issues denounced and mocked by others. It wants its own opinions to set the national tone. It wants them to prevail, to wield cultural and social power in a way they believe they once did and no longer do. Which is another way of saying that the anti-PC crowd is primarily animated and provoked by a sense of defeat, failure, and cultural grievance. It's an angry reaction of people feeling themselves on the losing end of an argument.

But what exactly is the argument about?

Here's an instructive example. My wife grew up in rural eastern Ohio. Until she left for college, she had met only a single non-Christian (her Jewish pediatrician) in her entire life. At college on the East Coast, roughly one third of the student body of 5,000 undergraduates was Jewish. When she returned home to Ohio for winter break, she reprimanded her father when he wished a neighbor "Merry Christmas." It would have been more considerate to say "Happy Holidays," she insisted.

Was she a crusader for tolerance among narrow-minded bigots or a commissar of political correctness brainwashed in three short months by a cadre of liberal elites? The answer, I think, is neither. In the context of her hometown, my future wife was being … overly sensitive. The chance of offending a resident of the town by wishing him "Merry Christmas" was vanishingly small. But on her much more diverse college campus, adjusting expectations and word choice to avoid saying "Merry Christmas" to a non-Christian made sense.

Or did it? Another component of anti-PC ideology is its denigration of concerns about social sensitivity. A Jew or a Muslim or an atheist taking offense at being presumed a Christian? That's just pathetic whining.

Widespread concerns about insensitivity and giving offense are relatively new in American life. From signs proclaiming that "No Irish Need Apply" to joke books published as recently as the 1980s that were little more than compilations of ugly stereotypes of Italians, Poles, blacks, Jews, Hispanics, gays, and other groups, the U.S. managed to muddle through much of its history with everyone except for white Anglo-Saxon Protestants having to endure mockery at the hands of everyone else.

But over the past few decades, as the country has developed a more multicultural common culture, national institutions that tend to be based in diverse, cosmopolitan cities — especially the federal government and mainstream media — have attempted to foster greater social cohesion by promulgating a civic ethic of tolerance and sensitivity regarding an ever-lengthening list of potential points of tension or difference.

The consequence? For one thing, it's probably much less likely than it once was that a Jewish kid or black kid or Puerto Rican kid or gay kid will have to endure being called any number of slurs. All of that was still commonplace when I was growing up and attending public schools in the early and mid-1980s. It's far less common today, and for that we can thank political correctness — which for those of us who live in diverse communities amounts to nothing more objectionable than the background presumption that we will treat each other with a minimum of respect that includes the avoidance of gratuitous insults and other forms of social cruelty.

But what about the people who live in places that remain as homogenous as they were decades ago — and who very much want it to remain that way? When my wife returned to her hometown in 1988, her reprimand merely provoked an eye-roll from her parents. Today her town is still overwhelmingly white and Christian. The nearest gay bar is more than 30 minutes away on the interstate. Yet the government and mainstream media continually push the message that mores need to change to reflect a country filled with blacks and Jews and Latinos and Muslims and gays and the transgendered. To the white Christian residents of rural eastern Ohio, this sounds like unnecessary moral fussiness mixed with condescension and backed up by the threat of coercion.

When your 18-year-old daughter implies you're an insensitive jerk, it's mildly annoying. When some of the most powerful and far-reaching institutions in American life make that case over and over again for years on end about an ever-expanding list of issues — well, that's enough to turn a town full of voters into Trumpkins.

Just as the European Union's attempt to do away with forms of national solidarity is (paradoxically) inspiring a resurgence of nationalism across the continent, so the understandable drive to forge a multicultural America committed to affirming differences has sparked an equal-and-opposite anti-multicultural backlash.

That's what the opposition to political correctness is all about — and one powerful reason why, despite the dramatic ups and downs of his quixotic campaign for president, Donald Trump has managed to rally so many to his cause.