Investigative journalist Jonathan Greenberg tells a long detailed story in the Post about Young Donald Trump’s obsessive campaign to get himself onto the Forbes 400 list by means of prolific lies about how many $$$ he really had.

In May 1984, an official from the Trump Organization called to tell me how rich Donald J. Trump was. I was reporting for the Forbes 400, the magazine’s annual ranking of America’s richest people, for the third year. In the previous edition, we’d valued Trump’s holdings at $200 million, only one-fifth of what he claimed to own in our interviews. This time, his aide urged me on the phone, I needed to understand just how loaded Trump really was.

The official was John Barron — a name we now know as an alter ego of Trump himself. When I recently rediscovered and listened, for first time since that year, to the tapes I made of this and other phone calls, I was amazed that I didn’t see through the ruse: Although Trump altered some cadences and affected a slightly stronger New York accent, it was clearly him. “Barron” told me that Trump had taken possession of the business he ran with his father, Fred. “Most of the assets have been consolidated to Mr. Trump,” he said. “You have down Fred Trump [as half owner] . . . but I think you can really use Donald Trump now.” Trump, through this sockpuppet, was telling me he owned “in excess of 90 percent” of his family’s business. With all the home runs Trump was hitting in real estate, Barron told me, he should be called a billionaire.

Greenberg suspected some of that was untrue, and he did a lot of poking, and was proud for years of the job Forbes had done calling Trump on his distortions.

But it took decades to unwind the elaborate farce Trump had built to project an image as one of the richest people in America. Nearly every assertion supporting that claim was untrue. Trump wasn’t just poorer than he said he was. Over time I have learned that he should not have been on the first three Forbes 400 lists at all. In our first-ever list, in 1982, we included him at $100 million, but Trump was actually worth roughly $5 million — a paltry sum by the standards of his super-monied peers — as a spate of government reports and books showed only much later.

That’s Trump for you – he tells so many and such huge lies that he gets away with many of them because people don’t realize they’re only at the first level. It’s worked, in a sense, but at the cost of being a known asshole to anyone with working radar.

I was a determined 25-year-old reporter, and I thought that, by reeling Trump back from some of his more outrageous claims, I’d done a public service and exposed the truth. But his confident deceptions were so big that they had an unexpected effect: Instead of believing that they were outright fabrications, my Forbes colleagues and I saw them simply as vain embellishments on the truth. We were so wrong.

This was a model Trump would use for the rest of his career, telling a lie so cosmic that people believed that some kernel of it had to be real. The tactic landed him a place he hadn’t earned on the Forbes list — and led to future accolades, press coverage and deals. It eventually paved a path toward the presidency.

It also got him the fortune he does have, because people believed enough of his lies that they wanted his name on their buildings, so now he gets to make millions just as a brand.

Trump knew I had doubts about his assertions, so he had his lawyer, Roy Cohn, call me. Cohn spent most of his time threatening lawsuits, schmoozing with mobster clients and badgering reporters with off-the-record utterances that made his clients look good and their enemies look bad. Cohn surprised me at my Forbes desk that summer: “Jon Greenberg,” a scrappy voice bellowed, before I could connect my tape recorder. I took notes by hand. “This is Roy. Roy Cohn! You can’t quote me! But Donny tells me you’re putting together this list of rich people. He says you’ve got him down for just $200 million! That’s way too low, way too low! Listen, I’m Donny’s personal lawyer, but he said I could talk to you about this. I am sitting here looking at his current bank statement. It shows he’s got more than $500 million in liquid assets, just cash. That’s just Donald, nothing to do with Fred, and it’s just cash.” He concluded: “He’s worth more than any of those other guys in this town!”

I offered to have a messenger pick up the bank statement at his office. Cohn protested that the document was confidential. “Just trust me,” he said. I told him I wouldn’t take his word without seeing the paperwork. “It’s confidential!” Cohn yelled.

So, to sum up, Cohn calls up this investigative reporter to say Trump is MUCH richer than the reporter thinks, and he has the documentary evidence right there in his hand, and no way is he going to show it to the reporter, because it’s confidential, man.

What self-respecting reporter would not take Roy Cohn’s word for it? Isn’t that what reporters do: take people’s word for things? Especially contested things, things that are the very core of the reporting and that the crooked lawyer is calling them up about?

I was a leading New York real estate reporter through the 1980s. I left the Forbes staff in 1983 but continued to freelance for the magazine while writing major investigative features as a contributing editor for the new Manhattan, Inc. magazine, as well as New York, Avenue and New York City Business. I knew all the key players. I thought I had a handle on this material.

But Trump was so competent in conning me that, until 35 years later, I did not know I had been conned. Instead, I have gone through my career in national media with a misinformed sense of satisfaction that, as a perceptive young journalist, I called Trump on his lies and gave Forbes readers who used the Rich List as a barometer of private wealth a more accurate picture of his finances than the one he was selling.

The joke was on me — and everyone else. Trump’s fabrications provided the basis for a vastly inflated wealth assessment for the Forbes 400 that would give him cachet for decades as a triumphant businessman.

Which is how he got to play one on tv and thus how he got to be president.

“The more often Forbes mentioned him, the more credible Donald’s claim to vast wealth became,” O’Brien said, arguing that Trump and the list were “mutually reinforcing”: “The more credible his claim to vast wealth became, the easier it was for him to get on the Forbes 400 — which became the standard that other media, and apparently some of the country’s biggest banks, used when judging Donald’s riches.”

He wasn’t wrong to be hell-bent on getting onto the Forbes list; it worked for him.

(Also, tip of the hat for naming his son “Barron.” The kid is named after a gigantic fraud. That will be nice for him.)