The other GOP candidates are finally starting to fight on Donald Trump’s terms, to judge by the slugfest that was Saturday night’s debate. But it speaks volumes — whole encyclopedias — about the ignorance of our political and media elites that they’re only now realizing that much of what Trump’s been doing is just busting balls.

It’s a blue-collar ritual, with clear rules — overtly insulting, sure, but with infinite subtleties. It can be a test of manliness, a sign of respect, a way of bonding and much more.

Rule No. 1: You can wince, but don’t squeal.

Rule No. 2: Bust right back, if you can.

Not knowing how to play is no excuse. And not getting it shows you have no idea how a huge swath of America lives — the Americans whom Trump has made his base.

From the start, Trump targeted the (mostly) white working class, which happens to be 40 percent of the country. And he’s done it not just with issues, but with how he talks — the ball-busting, the “bragging,” the over-the-top promises.

“Bragging” is in quotes because it’s not (all) about his ego: The endless reciting of poll numbers, the constant references to how much the media’s paying attention, is mainly about showing that he’s beating the cultural elite.

Beating the elite on behalf of his voters — who’ve been invisible to the politicians and the media for decades.

Consider a huge story that vanished almost immediately in early November: Two Princeton economists discovered that deaths are soaring among middle-aged, low-education whites.

The rise in mortality from 1999 to 2014 was 22 percent: Up 134 deaths per 100,000 for whites aged 45 to 54 whose education ended in high school.

To blame: jumps in suicides and in deaths from drug abuse — that is, from alcoholic liver disease plus overdoses of heroin and prescription opiates.

One of the economists, Nobel winner Angus Deaton, notes that the only modern trend that compares is the AIDS epidemic.

AIDS won headlines for a decade. The Deaton findings basically vanished from the media after a day.

And these soaring death rates are just one sign of the stresses the American working class faces. Many other blue-collar folks struggle on OK. But they know they’ve got huge problems that just don’t get talked about — and anyone who does raise them gets denounced and then ignored.

Until Trump.

America hasn’t been great for the working class for decades — which is why “Make America Great Again” is a great slogan for a guy who’s talking tough on the problems that blue-collar Americans (and more than a few middle-class folks) see as killing them.

And getting attention — unbelievable attention — even as he breaks all the “establishment” rules.

Because he’s playing and winning by blue-collar rules, and what are you gonna do about it?

Of course, he also has to show he’s for real. Why should these voters trust him? Answering those unspoken doubts is why Trump doubles down on his promises — he’s not just going to build a wall, but make Mexico pay.

First off, it’s funny — always a plus. More, it says he’ll damn well at least get the wall built.

It’s the same for the huge tariffs on Chinese imports and so on: Politicians never deliver everything they promise, but the higher they aim, the more likely you’ll actually get something.

And if you start negotiations with a reasonable offer, you wind up with President Obama’s lousy Iran nuclear deal.

Even Republicans and conservatives who don’t agree on a given issue appreciate Trump’s approach. Partly, it’s that — unlike the pundits and the Washington consultants — they’ve got working-class folks in their towns and in their families. They know how tough it is.

Plus, they’re tired of losing, too.

Tired of seeing their “leaders” talk and play by the liberal media’s rules, giving away the game before it even starts.

There’s a lot of screaming about how Trump’s ignored all the rules, lowered the level of debate, etc. But that’s because he’s playing by his own rules — and far from alone in that, considering his poll numbers.

There’s this, too: Every new president since the ’70s (at least) has been the guy who invented a fresh way to win.

I closed a column last February with this:

“A winning Republican has to connect with regular Americans in ways that call out the liberal establishment. The New York Times and the networks always called Reagan an extremist. He kept right on talking past their noise.

“His ability to do that was what they really hated about him. America needs a Republican they’ll hate every bit as much.”

I was thinking of a regular Republican who could establish real working-class appeal. Trump has instead started with the working-class appeal, and built outward from there.

Which sure sounds like a new way to win.

Now watch all the times Donald Trump has ejected people from his campaign rallies: