Tony Schwartz spent the best part of a year trailing Donald Trump when he ghostwrote the tycoon’s 1987 bestseller The Art of the Deal.

He believes he knows him well enough to predict that if Mr Trump is elected president, he would try and impose martial law, attack the free press and launch an attack on people he felt had slighted him that could make Richard Nixon’s actions against his enemies appear like "child’s play".

“I started out saying … that my highest fear was that because he was so thin-skinned, and because he is so insecure, he is a huge risk to set off, to punch in, the nuclear codes, because he happens to be irritated or frustrated by an enemy,” Mr Schwartz told The Independent.

Mr Schwartz said he was embarrassed by his work with Mr Trump (PBS)

“When I said that, I got a lot of rolling of the eyes from people in the media and other people to whom I was making that case. I think today, people do really begin to understand that this is a volatile man with very low self control.”

Mr Schwartz first broke his silence and revealed his views about what he considered Mr Trump’s unsuitability for the White House this summer, in an extended interview with The New Yorker. At the time, he expressed sadness about helping produce a glowing portrait of the businessman, that was in reality a work of fiction. He said he was obliged to “put lipstick on a pig”.

“I feel a deep sense of remorse that I contributed to presenting Trump in a way that brought him wider attention and made him more appealing than he is,” he said.

Since then, Mr Schwartz has been an outspoken thorn in the Republican presidential candidate’s campaign, launching scathing assaults from his Twitter account and appearing frequently as a guest on television panels.

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On Friday, he will be speaking to students at Oxford University, with an address entitled “Into the belly of the beast: How Donald Trump led me on the path to dharma”.

But if Mr Schwartz can claim to have found a position of Zen calmness, on the topic of the man he once trailed, he can sound impassioned, emotional and even worried.

Most polls and simulations suggest that Mr Trump faces a very tough battle to beat Ms Clinton, but if he does make it to the White House, Mr Schwartz said he believed three things would happen very quickly.

“On day one he would end a free press,” he said, speaking from Washington DC. “In any way that he could, he would use the government to shut down a free press, and listen, he has plenty of precedents for doing that, including his hero Vladimir Putin.”

He said he believed Mr Trump would then “conduct an ‘enemies’ campaign that would make what Nixon did in the Sixties and early Seventies look like child’s play”. He said he would go after every person he felt had wronged him in the the most “intense way” he felt he could get away with.

Donald Trump said he'd love to fight Vice President Joe Biden

He added: “I think before very long, its quite possible that he would find a way to declare martial law.”

Asked how Mr Trump would go about undertaking such a drastic measure, he said many of Mr Trump’s supporters were police, members of the border guards force and the “far right wing” of the military.

“He controls the levers of powers. There is nobody standing between him and punching those nuclear codes other than the guy standing there is who is obligated to do what he is asked to do,” he said.

“Just look at any country that has been taken over by the military. He’d say there is a threat to the republic and the military needs to crack down and he would start with curfews, and the stop and frisk of anyone who is not white, male and rich.”

Mr Trump's campaign did not respond to inquiries. However, the tycoon has over the months dismissed Mr Schwartz’s comments, saying on Twitter: “Dummy writer @tonyschwartz, who wanted to do a second book with me for years (I said no), is now a hostile basket case who feels jilted!”

On another instance, he wrote: “I haven’t seen @tonyschwartz in many years, he hardly knows me. Never liked his style. Super lib, Crooked H supporter. Irrelevant dope.”

Asked if he was being paranoid about the dangers of a Trump presidency, Mr Schwartz said: “There is that phrase, just because you’re paranoid does’t mean people aren’t out to go you. I think it’s a healthy paranoia.”

He added: “It’s just a deep knowledge of who this human being is, and a recognition … that he is a classic sociopath, the most critical quality of a sociopath is an absence of conscience. It is a reflection of deep inner emptiness and the way to fill that emptiness is through power.”

He said that however extreme and bleak his comments sounded, they were not made as someone who was crazed or out control, but as a “trained observer of human beings”.