The first time I interview Mark Singer it’s the Tuesday after the Republican national convention, which culminated with a Donald Trump speech Singer accurately describes as “scripted insanity”. Things feel pretty bleak. Trump’s candidacy, says Singer, is “the most cynical and truly sad thing that’s ever happened in my life in this country, and I’ve lived through assassinations and terrible wars, but that it has led to this – it’s a mixture of every awful emotion. If we elect this man it’s the end of a lot of things, I just don’t know how that happens. With a whimper perhaps – I hope not a bang.”

Born in 1950, Singer has been a writer for the New Yorker since 1974 (“There was no reason to leave,” he tells me as we sit outside a café near his home on Manhattan’s Upper East Side). In 1996, his then editor Tina Brown commissioned him to write a profile of Trump with the words “He’s totally full of shit, you’ll love him!” After months getting a full immersion into Trump’s world, one highlight being a ludicrous meeting between Trump and Aleksandr Lebed, in which Trump proudly showed the Russian general and Kremlin fixer a shoe he’d been given by Shaquille O’Neal – Singer wrote a 10,000-word profile that nailed the narcissism, superficiality and cynicism with which the world is now so alarmingly familiar. It concluded that Trump had “aspired to and achieved the ultimate luxury, an existence unmolested by the rumbling of a soul”.

Naturally, Trump didn’t appreciate Singer’s efforts, attacking him in his book Trump: the Art of the Comeback, in a letter to the New York Times (“he was not born with great writing ability”), and finally in an irate missive that read: “Mark, you are a total loser! And your book (and writings) sucks!”

These last two sentences are proudly printed on the back of Singer’s new book Trump and Me, which revisits his profile in the light of Trump’s presidential campaign. Given Singer’s probing wit and Trump’s essential ridiculousness, it is very funny, though the implications of a Trump presidency seem increasingly less amusing in the light of the hatred he has unleashed. “I am afraid. Everybody I know is afraid,” Singer says. “If Hillary Clinton wins, listening to the sigh of relief just on this island, we might have a carbon dioxide overload crisis. You’ll see these trees shooting up.”

Singer says that Trump is “the most unapologetic … there has to be a word stronger than hypocrite, there really does.” He is despairing that some voters – stoked by the email scandal and the WikiLeaks revelations about her attempts to undermine Bernie Sanders – think Hillary Clinton is more dishonest than Trump. This, he says, “is what really makes you want to blow your brains out. It makes you want to go back and find patient zero and think, when was the moment where we stopped being willing to fund public education in this country, that it became so egregious that we no longer could have people who understood the constitution, the checks and balances? Trump says: ‘I love the ill-educated.’ You bet he does.”

Singer blames Fox News and the web for what seems to be a growing disregard for the truth, but also acknowledges that widening inequality, leaving a large swathe of Americans essentially abandoned by the two main parties, has helped account for Trump’s rise – along with “the haters”, racists and xenophobes excited by this promises to keep Mexicans and Muslims out of America.

Nevertheless, he can’t bring himself to picture Trump actually being elected. “Imagine November 9, 2016. You wake up and there it is in the newspaper, Donald Trump has been elected president. He’s not going to institute martial law yet, he’s not going to abridge the first amendment just yet, he’s not going to seal the borders – he can’t do anything until January 20. This will be the beginning of a series of anni horobili that we’ve never seen.”

Singer cites a recent obituary of a friend’s aunt, which concluded: “Died of complications of congestive heart failure and the 2016 presidential election.” Like her, he says, “I think that you’re going to see a lot of people who’ve been hanging on, letting go. I believe that. And a lot of them are going to be envied.”

‘Trump in his quintessence’

Just a week later, however, things look very different. Trump is plunging in the polls, largely thanks to his typically impetuous and politically suicidal decision to attack Khizr and Ghazala Khan, the parents of US army captain Humayun Khan, who was killed in the Iraq war. “You have sacrificed nothing,” Khan said of Trump in his speech at the DNC, a line devastating because it is so self-evidently true, and one to which Trump was stupid enough to respond, telling ABC: “I think I’ve made a lot of sacrifices.”

“That almost made me embarrassed for humanity,” says Singer when I call him a few days after Trump’s car-crash performance. “I don’t really lump myself in the same humanity that possibly he belongs to.”

Singer says the Khan episode “showed Donald Trump in his quintessence”. Even the rightwing political commentator Bill O’Reilly, he says, “who’s a bomb thrower”, told Trump on his Fox show that you don’t attack a gold-star mother. “Trump is saying: ‘Well yeah, but’ – and then he says, as plain as day, ‘I didn’t attack her.’ And right after he says that he says: ‘I was viciously attacked.’ This demonstrates as plainly as anything that he’s a person who lies reflexively.”

Now Trump seems to be flailing. “He’s shocked,” Singer says. “He’s in a tailspin because his ego is constructed in a way that does not brook any kind of disagreement. He has his own party telling him, ‘Dude, shut the fuck up. Do not go there.’ These are people like Rudy Guiliani and Newt Gingrich, this is not Nancy Pelosi, OK? He hears them, he goes out and the next day does it again.”

This reckless behaviour has had many commentators wondering whether Trump is suffering from a mental illness. Last week, Singer participated in an extraordinary discussion on MSNBC’s The Last Word With Lawrence O’Donnell with Dr George Simon, a psychologist described on his website as “the leading expert on manipulators and other disturbed characters”. To be diagnosed with a narcissistic personality disorder, you need to exhibit five of around a dozen symptoms on a checklist, such as “Has a grandiose sense of self-importance” and “lacks empathy”. However, the entire list seemed to apply to Trump.

“His mental illness is now the conversation,” Singer says. There are tricky ethical issues around such a long-distance diagnosis – not least, he says, the fact that comparing them to Trump may further stigmatize sufferers. “It’s extremely unfair to people who have various mental illnesses.”

Singer says that even mental illness can’t entirely account for Trump’s behaviour. “You can’t believe that this is somebody capable of this degree of obtuseness, obliviousness, total absence of self-awareness and cruelty. This combination. Since you and I spoke the other day, that has blown open and he’s really done. So now the headlines are, what’s the party going to do? They’ve got to do something. They’re almost conceding the election now, two weeks after their convention!” Singer even made a bet last Monday with Jane Mayer, a New Yorker colleague, that Trump would be gone in two weeks. “The wheels are starting to come off so dramatically and you can feel it.”

Given his plunging popularity, Singer says it’s no surprise that Trump is now claiming that the election will be rigged. “It’s in the book,” he points out. “The thing that he’s dreaded most in his life is being labeled a loser. So he has to have an explanation for that.”

In the shorter term, Singer says, Trump’s next big challenge will be the debates – or more specially, how he can avoid them. As Trump has demonstrated many times, for instance recently claiming that Russia would not invade Ukraine, when Russian soldiers occupied Crimea over two years ago, when it comes to events beyond America’s borders he is profoundly ignorant. “Exactly right,” Singer says. “By choice and temperament both. And this is a person who brags: ‘I know more about Isis than the generals.’ This party has this quote on record when they nominated him. Guess who I don’t have a whole lot of sympathy for? It’s game over.” Given the warp speed at which the news has moved this year however, Singer rows back: “That is the kind of stuff that is so dangerous to say I should bite my tongue.”

Trump, says Singer, is the most transparent politician there has ever been: “He’s only out for himself, in every way, in every gesture.” One of the reasons for publishing his book, he says, was in disgust at the way journalists indulged Trump’s race for the presidency because it was good for ratings and traffic. “This notion of false equivalency is not something that journalists have figured out, but it wasn’t even that with Trump, it was just the celebrity and [him] using up all the oxygen. But I knew that Trump had no intention of winning, it was obvious – and here we are.”

As for many Americans, the Trump candidacy has been a draining experience for Singer. No one can turn away, Singer least of all. “I just sat here and I was watching MSNBC last night until 11 o’clock and I realized, I am now an addict. I’m totally a junkie and I really need to have an intervention of my own, I’m afraid.”

That sets him off on another Trumpian train of thought – those in the Republican party who think they can perform some kind of intervention. “This idea of doing an intervention with Trump is hilarious to me. These might be the last optimists in America.” You can’t treat people like Trump, he says. “They’re incurable.”