The sense of foreboding around Donald Trump’s presidency heightened perceptibly this week following verdicts in courts in Virginia and New York. Trump’s former campaign chairman, Paul Manafort, was found guilty on multiple charges of tax evasion and bank fraud, while his former personal lawyer, Michael Cohen, pled guilty to similar crimes, also admitting under oath that he had carried out illegal campaign financing under the president’s direction to silence women who claimed to have had affairs with Trump.

Slowly, but surely, the veil is being lifted on the squalid and potentially criminal business dealings of the president and his associates. But normally sober and dispassionate observers are also discussing something even more extraordinary. Since Trump’s performance at the Helsinki summit last month, when he abased himself before Vladimir Putin, analysts have been openly contemplating the possibility that the United States has elected a Russian agent as president.

On the face of it, the idea still seems preposterous. Ever since the 1962 film The Manchurian Candidate depicted a Russian plot to install a brainwashed former POW in the White House, the idea has been confined to the more cartoonish sort of blockbuster movie or to the outer circles of conspiracy theory nuttiness, ranked alongside 9/11 Truthers and those who believe Barack Obama is a secret Muslim.

According to Craig Unger, Donald Trump has been cultivated, developed and exploited by Moscow since before the fall of the Soviet Union

But the theory that Donald Trump is deeply, irredeemably entangled with Russian interests is no longer on the fringe – it forms the substance of the Steele dossier drawn up by a former MI5 agent and passed to the FBI in 2016. It’s a subtext of independent counsel Robert Mueller’s investigation into Russian interference on Trump’s behalf in the 2016 election.

Former head of the CIA John Brennan, whom Trump stripped of his security clearances last week, has been dropping heavy hints that he has seen deeply damaging classified intelligence on the president, whom he has accused of treason. And it has been a recurring leitmotif in a number of in-depth journalistic investigations into Trump and Russia carried out by publications such as the Washington Post, the New York Times and The New Yorker.

Craig Unger’s new book, House of Trump, House of Putin: The Untold Story of Donald Trump and the Russian Mafia, stitches together all of these strands and goes one further.

According to Unger, Donald Trump has been cultivated, developed and exploited by Moscow since before the fall of the Soviet Union. Russian intelligence agencies probably have compromising sexual material on him going back more than 30 years; they have used him as a funnel for laundering dirty money from the Russian mafia and from post-Soviet oligarchs, and have more than enough compromising material on Trump to bend him to their will.

“I believe frankly that what we’re witnessing has been the greatest intelligence operation of our time,” says Unger, a veteran journalist for the New Yorker, Esquire and Vanity Fair, who 14 years ago published House of Bush, House of Saud, about the links between the Bush family and Saudi Arabia.

Unger believes this is a crisis that’s been decades in the making. “But you end up with a Russian asset in the White House – the president of the United States.”

While Trump has repeatedly asserted that he has had “zero contacts” with Russia, Unger says he can name at least 59 people who were intermediaries between the president and Russia over the last 30 years.

“It starts off with the arrival of the Russian mafia in New York in the late 1970s and early 1980s, and a guy named David Bogatin, who meets with Donald Trump,” he says. “He buys five condos for six million dollars. That’s the equivalent of about 15 million dollars today. And that’s sort of the beginning.”

Listen to Craig Unger in conversation with Hugh Linehan

According to Unger, since that first deal at least 1,300 Trump condominiums have been sold to Russian-connected buyers. “A cheap Trump condo costs over a million dollars,” he says. “So we have over 1,300 condos that meet all the criteria for what is normally called money laundering. By that I mean these were purchases by anonymous shell companies and they were all cash transactions. What is happening is that the Russian mafia is using Trump real estate to launder money.”

Trump’s psychological profile could hardly be better tailored to being easily turned by a hostile intelligence agency. ‘We’re talking about a guy who is susceptible to flattery’

According to the book, over the years a two-track process took place. “On the one hand the Russian mobsters are in New York and they’re starting to launder their money through Trump properties and some of them are actually taking up residence in Trump properties,” says Unger. “On the other hand – this is before the fall of the Soviet Union – you see the Soviets approach Donald Trump in 1986 and 1987. And you have the Soviet ambassador and his daughter, who meet with Trump and say how marvellous Trump Tower is. They want him to do stuff in Russia.

“I interviewed General Oleg Kalugin, the former head of counterintelligence for the KGB. He was saying typically that’s how it starts, with an innocent conversation. Then they flew Trump over to Moscow. Kalugin said that he believed that Trump had fun with lots of girls during that trip and he was almost certain that the KGB had kompromat [compromising material] on that.”

Certainly, the psychological personal profile of Donald Trump could hardly be better tailored to being easily turned by a hostile intelligence agency.

“Absolutely,” Unger agrees. “We’re talking about a guy who is susceptible to flattery. He loves it when people say nice things about him.”

Over 30 years, Unger says, there was one criminal operation after another involving the Russian mob and Trump. “I believe these are not random and these are not coincidences. The mafia plays a very different role in Russia than in the United States.”

There are some pretty nasty people involved in all this. Thuggish mobsters and decadent oligarchs abound. One recurring character is Semion Mogilevich, depicted in the book as the most powerful organised crime figure in Russia.

Unger says he has transcripts of secret recordings of Ukrainian intelligence in which they talk about Vladimir Putin’s meeting with Mogilevich. “That is a matter of fact that I think can’t be disputed,” he says.

He also says General Kalugin told him that the mafia in Russia is effectively a state actor. “When you unpack that a bit and you look at the people who’ve been in Trump Tower you realise that there have been Russian mafia operatives who have links to Russian intelligence in the home of the man who is now president of the United States. I find that very disturbing.”

When Trump ran into serious financial trouble in the 1990s, he became even more indebted and vulnerable, says Unger. “He ended up four billion dollars in debt, and it was the Russian mafia who came to his aid. They own him. They brought him back to life. He owes them. And when people say well where’s the smoking gun, where’s the kompromat. I think it’s staring us there in plain sight.”

House of Trump, House of Putin depicts a tangled, often confusing world of shady deals and shadier people. Many of the protagonists from Russia or other post-Soviet republics are known by two or three different names, which adds to the confusion. It’s a sometimes dizzying blizzard of dodgy deals, quasi-legal contracts and dubiously sourced money.

On the face of it, it looks like a huge dumpster fire of sometimes blatant criminality. But, as with everything in Trumpworld, it’s difficult to pin down the exact truth. Unger does a good job of synthesising the work of other writers (all of them credited) on these many different, troubling strands, but does not fully deliver on the “untold story” promise of the book’s title.

I think they certainly cultivated him. Every time he came back from Russia he started talking about running for president

One aspect that is hard to reconcile with the overall thesis is this; if Donald Trump is a Russian asset, why is he so terrible at concealing it?

“Well in fact that was the hardest thing for me to believe when I read the Steele dossier,” Unger agrees. “It alleges that Trump is funnelling intelligence to the Russians. I thought that doesn’t make sense at all. It’s hard to come up with anyone who’s less disciplined as a spy than Donald Trump.

“But one of the things I’ve began to realise about this is that when he had those Russians in Trump Tower they could be gathering intelligence on the number of condos they’re selling to various oligarchs; they know who is financing these new Trump branded developments and they can easily funnel that kind of intelligence to the Kremlin.”

Unger agrees, though, that the Russians almost certainly never expected their man to become president.

“People are saying how did they know 30 years ago that he was going to be president,” he says. “The answer of course is they didn’t. When I talked to General Kalugin he said that they had at least 300 American spies in that era before the fall of the Soviet Union.

“But I think they certainly cultivated him. Every time he came back from Russia he started talking about running for president. There was a full page ad that he took out in the New York Times, the Boston Globe and the Washington Post in 1988, putting forth these foreign policy points, and it could have been dictated by Vladimir Putin today. It was an assault against Nato, against Europe, ‘we’re wasting our money being friends with Europe’ and so on.”

If Unger is correct and the president of the US is an agent of a foreign power, it would be the worst scandal in American history. How does he think this will all end?

“I think and hope that he will be impeached. Whether it leads to an indictment or a conviction, I do not know. I have to think the spectacle that will unfold will be pretty devastating and we’re just at the beginning of that right now. These are early days still.”

House of Trump, House of Putin: The Untold Story of Donald Trump and the Russian Mafia is published by Bantam Press. Hugh Linehan talks to Craig Unger on the Irish Times Inside Politics podcast