Mark Singer is a staff writer at the New Yorker. In 1996, he was asked by its then editor, Tina Brown, to profile Donald Trump, an assignment that involved spending weeks in the property tycoon’s company. Following Trump’s successful campaign to be the Republican party candidate for president, Singer has now revisited these encounters in a hilariously scathing book, Trump & Me. On its jacket is a blurb from its subject. It reads: “Mark, you are a total loser! And your book (and writings) sucks!”

Do you think Trump can win?

No, I don’t. I was worried about it two or three weeks ago. But he has shot himself in the foot since then. Attacking Gonzalo Curiel [the US district judge who is handling the Trump University lawsuit] was an unforced error, a stupid thing to have done. There are four and a half months to the election, and he will do this kind of thing again and again. He is shameless. People at his rallies seem convinced this guy is going to build this wall [between the US and Mexico]. It’s a metaphor, but they think it’s real. And since he is shameless, he will encourage these illusions. But this notion that anyone can control Trump... He can’t be controlled. It’s obvious. He’s all id. So, he will continue to shoot himself in the foot. Also, Obama is unfettered now because he has endorsed Hillary. This is payback time for him. He is going to be Clinton’s greatest weapon.

But what if he does win?

Well, he is the commander-in-chief so that nixes the coup. But the National Security Agency and the CIA: these people are not going to go along with a Trumpian abnegation of civil liberties. There will be chaos of some sort. People say he’ll get elected and then impeached. That would be no fun, but it might be necessary. He has already done enormous damage to the United States. He has hurt it abroad. Every time he makes these anti-Muslim statements, he feeds the Isis narrative. This man is so self-involved, and now it’s layered with this extra megalomania because he is near real power.

Is the US press up to scrutinising him?

They’re rising to it now but, for a while, it was the most disgraceful performance. When Trump announced [he was running], people thought it would be great comedy, good for satire. I dissented from that. I knew how toxic he was. Given the tenor of the times, I knew he was not going to be good for the country. Anyway, it took a while for the factchecking [by the press] to kick in, and meanwhile he was overwhelming his opponents in debates by insulting them. They had no idea what to do: they were flummoxed, and so they fell one by one. The horse race became the narrative, and that lasted for months. No one was doing the reporting. But they’re doing it now. Trump University [against which there are fraud allegations] has been out there for years; his dealings with women, also. People need to look at his track record. His only real success has been branding himself.

In his introduction to your book, the editor of the New Yorker, David Remnick, traces Trump’s decision to stand for president to the 2011 White House correspondents’ dinner, when Obama made jokes about him. [In a dig at Trump’s obsession with his citizenship, Obama said he was ready to make his “birth video” public; he then showed a clip from The Lion King.] Do you agree that humiliation was the spur?

It’s not clear. In terms of his ego, that would mean he felt those jokes, and I doubt he can feel anything. But if you see the video of him, he’s seething. Watch it on YouTube. It’s terrific!

‘It never occurred to me that he would cash the cheque. He must have needed the money.’ Photograph: Richard Saker for the Observer

When Brown asked you to write about Trump, did your heart sink?

Yes, it did – although I thank her now. A friend of mine, another reporter on the magazine, said: ‘Just don’t do it.’ But Tina had a special drawer in her desk, and in that drawer was a jar, and in that jar were my testicles. [Singer had spent four years writing a book, and thus had not been writing many pieces for Brown.] I had to follow my testicles. Once I got into it, though, it was interesting to go into how he did things, and that’s when I realised the extent of his financial difficulties. [In the 1990s, Trump was in debt; among other things, he missed a $30m interest payment to one of the estimated 150 banks that were concerned about his financial wellbeing.] But he was too big to fail: that’s the simplest shorthand. He had over-leveraged himself, and the banks had to write off some of his debts, and then he just turned around and said: “I’m back and I’m better than ever.”

Later, your profile of Trump appeared in a collection of your work, a book that was reviewed positively in the New York Times. Trump responded to the review in a letter to the paper in which he said that you, unlike him, were “not born with great writing ability” and generally slagged you off. Your book shot up the Amazon chart and, as a token of your gratitude, you sent him a cheque.

Yes, for $37.82 (£26.74). And he cashed it. I was going to send him $1,000 because I wanted him to cash it. But I didn’t have $1,000. It never occurred to me he would cash this cheque. I sent it as a “fuck you”. People always want to know why he cashed the cheque and it’s obvious: he needed the money. [Laughs] You know, when he attacked me, I thought: I can die happy. I wrote about him again a couple of times, but it was only earlier this year that I decided to do this book. I guess I thought: Well, the country seems to be going down the toilet – what’s in it for me? A totally Trumpian thought.

Trump is arriving in the UK this week. What should we make of that?

I don’t know. It’s bizarre, coming here the day after the Brexit vote. But good luck! I guess you should think of him as someone who’s bringing back mad cow disease. That should do it.

Trump & Me by Mark Singer is published by Penguin on 5 July, £9.99. Click here to order a copy for £6.79