When the professor said the middle class comprised 90 percent of Americans, I told myself — I was too shy to challenge a professor out loud — he made it sound like we were all the same, without much room to be individuals.

Fifty years later, his 90 percent has dwindled, and instead of a preponderant middle class, we have, at best, a shrinking middle class. Some say it’s already gone. That may be the greatest threat to America. More than ISIS. More than Russia. More than China. More than Trump. Without the middle class to underpin our culture, we may be lost.

Social scientists break down the middle class into smaller chunks. Upper-middle class (professionals and folks who own businesses, the movers and shakers in town), the middle-middle (people who have good paying jobs that don’t require them to shower at the end of the shift), lower-middle (people with jobs in which they have to wash often).

With so many in the middle class, only a few folks were left over to populate an upper class, mostly of inherited wealth. And a small under class, folks on welfare and the Jed Clampetts of the world, up in the hills with a wisp of smoke rising from the chimney (or, “chimbley” to push the stereotype). But, the middle class was our bulwark. Our shining city on the hill. The American Dream was to enter it or to rise within or even above it.

The middle class came in for lots of ridicule. You can hear the scorn in “Pleasant Valley Sunday,” by the Monkees. “Rows of houses that are all the same, and no one seems to care” or Malvina Reynolds’s “Little boxes on the hillside, and they’re all made of ticky-tacky and they all look just the same.”

No imagination. No creativity. No diversity. Nothing of interest here, keep moving.

But still, inside those rows of houses that are all the same lived the heart and soul of the middle class. Nothing gets done in the United States without approval from the middle class. It is the ratifier. Or at least it was.

At the peak of the civil rights movement, white northerners saw injustice in the south and sat in and rode buses for freedom. Michael Schwerner. Andrew Goodman. Viola Liuzzo. William Louis Moore. The Rev. James Reeb. All white, all middle class. All died trying to help our country live up to its ideals. Polls showed again and again that huge chunks of the white middle class were on board with the civil rights movement.

But, things changed. A lot of white middle-class folks jumped ship, rightly or wrongly, when activist young blacks, impatient with the seemingly slow progress of non-violence, began to talk tough, though they seldom turned that talk into action.

Raised-fist salute. Black Power. H. Rap Brown. Black Panthers. Some of the black middle class joined in, too, especially the intelligentsia, such as LeRoi Jones, a poet and playwright who changed his name to Amiri Baraka. And upwardly mobile black leaders such as Malcolm X. The rhetoric of some black leaders repelled many, even Jews, who had been perhaps the strongest supporters. Blacks and Jews had shared a pretty solid rapport as denigrated people. But many Jews joined the rest of the white middle class and fled the civil rights movement and the cities, leaving both almost exclusively black.

The white middle class stopped ratifying civil rights goals. It was less the murder of Dr. Martin Luther King than this withdrawal of ratification that halted the movement. It has never regained momentum. A sign of this change is that last year a leader of Black Lives Matter said white people should support BLM but had no business joining.

The women’s movement walked a similar path, starting with broad support in the middle class and slowly frittering it away. When leaders began to pooh-pooh women who chose to work at home raising a family or who were stuck beneath the glass ceiling in dull office jobs as what we used to call secretaries, the end had begun for middle-class support of the movement. In which class, after all, were the work-at-home moms and secretaries?

The women’s movement probably hit its nadir when its “intellectuals” put it out that sex between a woman and a man is always rape. I doubt that squared with the experience of most of the middle class.

I grew up beneath the specter of the military draft. Everyone knew veterans who had served at the initiative of Uncle Sam, not their own. Many told horror stories about the military, but many others said the military “made a man of me.” And most of the middle class supported the draft.

Then came Vietnam. Thousands, maybe millions, avoided the draft legally (in school), quasi-legally (in the National Guard but MIA from training) or even illegally (falsifying a medical record with the help of a friendly doc who golfed with Daddy). These evasions undermined the credibility of selective service. The middle class withdrew its backing. In 1973, Congress ended the draft.

By any measure, much of the middle class is going, going, gone. The upper-middle is doing fine, as Robert Samuelson laid out in these pages last month. But those in the middle-middle are struggling to hold on, and statistics show they often lose the struggle. More and more white-collar jobs are being done by machines. You ever use the self-checkout? The 2016 election may have been the last gasp of the lower-middle class. Its jobs are not coming back. They didn’t emigrate, they died, crushed by robots.

America is more likely to die by suicide than by murder. The bulwark against that suicide for decades has been our middle class. Since the middle class was so huge, anything it ratified had popular support, by definition. From where will the support come for the next great social movement? Who will protect us when the middle class is gone?

Bob Neal isn’t sure whether he’s middle class. He has retired from farming and newspapering, neither of which paid well enough to leave his sons in inherited splendor.