At war with the US intelligence agencies he compares to Nazi Germany, damning them for leaking an unverified dossier on his alleged links with Moscow, Donald Trump’s bizarre press conference left the world agog. What if the Russians have so well destabilised America that no sooner inaugurated, their chosen president has to be impeached? Don’t count on it. Ordinary rules don’t apply to the man who is the raw spirit of the lawless wild west.

Polly Toynbee’s 1988 interview with Donald Trump | Polly Toynbee Read more

And Trump’s nature was never a secret. He has never dissembled, he can’t dissemble. Why would he when he worships every aspect of himself, each hair on his head, each word he tweets? Greater self-love hath no man.

Apart from his lost good looks, he is unchanged since I interviewed him for the Guardian back in 1988. He was 41 and in Britain to plug his book, The Art of the Deal. Then as now, he was a petrifying megalomaniac with no grip on reality, or not a reality shared by others. At the time I described his “demonic power and energy waiting to spring”. Now look how far he has sprung.

I wrote about his aura of “glitz, greed, glamour and an ambition so colossal that it will probably not rest until he rules the world – which one day he just might”. And next week, God help us, he will. But nearly 30 years ago was his eye already on running for the presidency? I put the question to him. “Not for a period but I am involved politically. You could do it from where I am,” he replied with the same nonchalance he might describe making a pitch for some new property or casino in New York.

Facebook Twitter Pinterest Polly Toynbee’s interview with Donald Trump in 1988

What would his platform be? “Respect. We’re a second-rate economic power, a debtor nation. We’re getting kicked around.” His current determination to tear up Barack Obama’s carefully brokered nuclear deal with Iran has a long history. He told me that as president, “I’d be harsh on Iran. They’ve been beating us psychologically, making us look like a bunch of fools … It’d be good for the world to take them on.”

How will Britain fare? Though his boastful book was a US bestseller, it had snooty reviews over here, as eminent British business people of the John Harvey-Jones type shuddered with repulsion at the cut-throat swagger of Trump’s style. In his corporate deals the other guy is always shafted, especially decent old-world types stupid enough to think a gentleman’s word is his bond.

Trump was in London in the midst of Margaret Thatcher’s big bang, red braces and loadsamoney mania years, yet his judgment on Britain was this: “Your country’s distaste for success is a national disease.” Bear that in mind as our government begs for a post-Brexit trade deal, with a Theresa May date not in Trump’s diary. What kind of trade deal does Liam Fox imagine he will broker with this man whose contempt for Britain, even back in its most Trumpish era, was so withering? Just as Trump’s view on Iran is unchanged, I doubt he has formed any new views about Britain.

Facebook Twitter Pinterest ‘My own assessment back then was: ‘He’s sharp as a gold-plated razor-blade.’’ Donald and Ivana Trump on their luxury yacht in 1988 Photograph: Marty Lederhandler/AP

The craven caperings of Nigel Farage and Boris Johnson may amuse him, but the deal will be cut-throat. If you had doubts about the Transatlantic Trade and Investment Partnership (TTIP), just wait and see what kind of deregulation, anti-working rights, anti-environmental, anti-product safety and food contamination rules he will impose when he shakes hands with British gentlemen on their knees for anything they can get.

Britain stares at him across the Atlantic, aghast at this grotesque man-baby, this loud-mouth buffoon and braggart, but my own assessment back then was: “He’s sharp as a gold-plated razor-blade.” Never underestimate the power of a person without self-doubt. Never underestimate the power of a gargantuan appetite for possessing everything. If he was frightening back then, he’s terrifying now.

That gigantic egotism is an Americanism with deep roots. Though not religious, Trump says he attends Marble Collegiate Church, home of the rightwing theology of Norman Vincent Peale – a pastor Ronald Reagan rewarded with the Presidential Medal of Freedom. I recognised in Trump the strong affirmation he drew from Peale’s bestseller The Power of Positive Thinking. Every atom of Trump’s being is imbued with the Peale magical thinking that says only believe in yourself (not God, not self-sacrifice) and you can take whatever you want in the here and now, not the hereafter. I can imagine Trump chanting to himself daily in front of the mirror the famous mantra, “Every day, in every way, I am getting better and better.”

When I met Trump, I had already heard Peale preach to a congregation of the rich, but addressing the poor too: just want it enough, and you can get it. If you fail that’s because you lack the necessary will, was a comforting message for the hyper-wealthy, confirming their meritorious deserts. To all the rest, the message was self-blame: you just don’t want it enough. That’s Trumpism.

Back in that 1988 interview I wrote: “Could Trump possibly make it to the White House? Of course not, says everyone who knows anything about American politics. It’s a bad joke. But then Trump has often done what can’t be done and if the White House can take a senile movie star, why not a casino operator?”

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That bad joke has landed. Here he is, transparent in all his hateful habits. So far those who voted for him show no sign of regretting it. What they saw is what they got. The more he transgresses, the more it proves the rules were “establishment” fixes to deny him his rights. Ethics? Conflict of interest? Everything is fake except him, authentic Trump whatever he does. If there is a sex tape and if it bursts on to the web, that might just cause a wobble – but why wouldn’t that just be fake too?

In foreign relations, a shockingly out-of-control and wildly unpredictable finger on the nuclear button may yield extraordinary results. Others may crumble before him when there are no rules, when mutually assured destruction is no longer a game of double and triple bluff but a possibility. He may confound the careful Chinese and outdo the North Koreans. My own hunch is that those dealing with him should beware of underestimating this man. There is method in his madness – and no one in the world is more primed to win anything and everything against any odds.

• This article was amended on 13 January 2017. An earlier version referred to “Peale’s famous mantra”; the mantra referred to was that of Émile Coué, whose ideas Peale later adopted.