The man who knows Donald Trump best believes that if Trump becomes president nuclear Armageddon could follow.

“I genuinely believe that if Trump wins and gets the nuclear codes there is an excellent chance it will lead to the end of civilization,” Tony Schwartz told The New Yorker this week.

Who is Schwartz? He is the writer of the phenomenally successful Trump memoir The Art of the Deal the book that truly put Trump on the national radar and kickstarted his rise to Republican nominee.

In the process of writing that book which sold over a million copies, Schwartz spent 18 months joined at the hip with Trump, camping out in his office, joining him on his plane and helicopter and spending weekends with him in Florida at his estate there.

He has never spoken of that time until now, and his opinion of Trump speaks for itself.

He says if he was writing the book now he’d change the title to The Sociopath.

“I put lipstick on a pig,” Schwartz now says, adding that he believes Trump is out of control, pathologically impulsive and self-centered.

Some believe Trump has Tourette Syndrome and blurts out insults even though he may not mean to, hardly the cool, calm disposition an American president should have.

This week's Trump takes the biggest stage of all at the Republican National Convention in Cleveland, and is one election away from the presidency. Schwartz is finally leveling on the man he turned into a hero for millions. He says he wrote the entire book and got half the proceeds but that Trump now actually believes that he wrote it.

When Schwartz began writing the book he soon realized one truth about Trump -- he had no attention span.

Schwartz tried to sit down with Trump over a series of weekends to tease out history, but after 10 minutes Trump would get fidgety and quickly cut the conversation short.

“It’s impossible to keep him focused on any topic other than his own self-aggrandizement,” says Schwartz. “If he had to be briefed on a crisis in the Situation Room it's impossible to imagine him paying attention.”

Schwartz says he “seriously doubts that Trump had ever read a book in his entire life.”

To make The Art of the Deal happen, Schwartz indulged in a dishonest practice of allowing Trump to be Trump by just following him around and eavesdropping on his phone calls to important clients.

Later when he interviewed Trump and the clients about what was being decided he discovered Trump had a completely different recall of what was said and promised.

Lying is second nature to him, Schwartz says. “More than any other person I have ever met, Trump has the ability to convince himself that whatever he’s saying at any given moment is true.”

He say Trump’s need for attention at all times is “completely compulsive…If he could run for emperor of the world he would.”

Schwartz freely admits that in The Art of the Deal he gussied up most of Trump’s faults successfully, so much so that Timothy O’Brien, who wrote a searing investigative book on Trump, said The Art of the Deal was a glittering fable which was a “nonfiction work of fiction.”

Schwartz also lifts the veils on Trump's six bankruptcies, saying he is amazed how he has still a stellar business reputation.

Finally, Schwartz warns those who think Trump will look out for them: “They will learn what anyone who deals closely with him knows -- that he could care less about them.”

Scary stuff.