Once upon a time, when I was editing this newspaper’s opinion pages, somebody decided that it wouldn’t hurt to have a slightly more cordial relationship with the Rev. Al ­Sharpton.

So there we were, in a Manhattan steakhouse, discussing not much because it quickly had become clear that Sharpton enjoyed being New York’s premier racial grievance-monger, so there was no common ground to be had.

But he did show a flicker of interest in a $350 cognac on the after-dinner menu — glancing over with amusement in his eyes as the waiter hovered. “If you do, reverend,” I said, “I’m asking for separate checks.”

Then came that towering thundercloud of a frown — meant to intimidate, as all too often it does.

I laughed. He screwed his face up tighter. I laughed again.

And then so did he, and presently the evening ended with a handshake and a personal lesson learned: If you push back at Rev, Al, he’s not so tough.

Not enough people understand that though — not enough see through the scam.

Al Sharpton has been making a good living for 35 years now, pushing New York and the nation as far as his bullhorn audacity will carry him — which is pretty damned far for a one-time FBI informer who started with nothing and brought even less to the table. He rode an eagerness to exploit racial discord, a couple of incendiary slogans and the cowardice of state, local and national leadership to fame and fortune.

How solid the fame is these days isn’t clear. Someone who has worked with him for decades said Monday: “[H]e has no influence in Atlanta, Chicago, anyplace but New York, and even that’s not certain.” Whether the fortune will survive all the tax liens also ­remains to be seen.

But one thing is clear as clear can be: If they ever build a Con Man Hall of Fame, Alfred Charles Sharpton Jr. must be an inaugural inductee.

Or, as President Trump put on Twitter Monday, “Al is a con man, always looking for a score. Just doing his thing.” This was in response to Sharpton’s criticism of Trump’s criticism of Baltimore and so on — Twitter being by nature somewhat circular.

And who’s to dispute the president’s ability to pick out a con man, he himself being so accomplished in the art — a New York City real estate developer and a successful politician, rolled up into a double dose of world-class flimflammery.

The difference is that Trump builds things; Sharpton tears them down.

Yet there’s a perverse symbiosis in the dispute — Sharpton playing to his base and Trump to his, never mind the merits for a moment — and not for the first time.

“I have known Al for 25 years,” Trump tweeted. “Always got along well. He ‘loved Trump!’ He would ask me for favors often,” — and, no doubt, vice versa. That’s how such relationships work — and clearly both benefited from them.

But time passes, circumstances change and now antagonism best suits their purposes — so, antagonistic they are.

On the merits, Sharpton was wrong. The rats in Baltimore are very real.

And Trump was dead right about both Baltimore and the political leadership in America’s large cities. Almost without exception, it embraces and endorses deterioration and decline — and if you doubt it, ride a New York City subway.

Trump is Trump — his rhetoric can be blood-curdling — but at the moment, he’s speaking truth to political power; Sharpton says he is, but that’s always been a con.

Bob McManus is a contributing editor of City Journal.