Pulitzer Prize-winning investigative journalist David Cay Johnston has been following Trump's career since 1988. In "The Making of Donald Trump," the former New York Times reporter and registered Republican calls the president-elect a 'dictator' and a 'deceiver' with no regard for the people who elected him.

DW: You have been quoted as saying there was "no way" Trump would win the election. How could you be so wrong?

David Cay Johnston: I regret it. Every bit of polling data at the time suggested that Hillary Clinton would convincingly win the popular vote, which in fact she did. The FBI director's statement about Hillary Clinton's e-mails 11 days before the election hurt her very badly. And larger numbers of white working class voters than expected turned out at the polls, people who believed Trump was going to relieve their economic situation. They bought his con job that it's the Chinese, the Mexicans and that "idiot" in the White House. And that he alone possesses the ability to fix this.

You have predicted a constitutional crisis and that Trump would be impeached within one year.

Trump does not even know what the president's job description is - the executive who carries out the will of Congress. But Trump talks like a dictator. He has said that he will order the military to kill women and children who are the families of people he believes are terrorists. Those would be crimes, and senior military officers will reject those orders. They would then be fired, or some other problem would arise. Trump has no respect for the Constitution. Remember, he even said he wouldn't accept a defeat.

Trump biographer David Cay Johnston

To be fair, his tone in his victory speech and in his interview on the CBS program "60 Minutes" was a bit more subdued. And during his White House visit, he looked respectful, even modest.

Having won, of course, he can sound reasonable and read remarks others wrote for him. But he is about to enter an unbelievable pressure cooker where he will know secrets that he cannot disclose for reasons of national security. He will then be criticized over what he's doing or not doing. Donald Trump is not equipped psychologically to deal with that.

Do you foresee Trump's business activities interfering with his duties as president?

This is a really important issue. Previous presidents had their stock and bond portfolios put into a blind trust. Federal officers are allowed to sell their materials without paying capital gains tax. Donald Trump's primary businesses involve long-term contractual relationships with Middle East potentates, Russian oligarchs and other enterprises around the world. Laws do not require complete disclosure. The national security and corruption implications of this are tremendous. How do we know if he makes a decision that isn't to prevent the collapse of one of his overseas businesses? This requires really close scrutiny.

As somebody who has been following Trump for decades, what are your biggest concerns about his presidency?

Donald Trump is manifestly unqualified to be president of the United States. He is a world-class narcissist who believes in his genetic superiority and greatness. Throughout his life, he has been defrauding workers or small business vendors and investors. He also has life-long associations with the American mafia, Russian mobsters, con artists, swindlers, violent felons and a confessed cocaine trafficker. He thinks that he can learn everything you need to know about missiles in 90 minutes.

All of these things together, especially his fragile and immature psyche, pose a grave threat to the world. I cannot see how this will end - except badly. But if he turns out to be a great president, I'll be the first person saying 'Man, did I get this dead wrong.'

David Cay Johnston is an award-winning journalist, educator and author of several books on tax and economic policy. "The Making of Donald Trump" is the culmination of nearly 30 years of reporting. It was released in the US in August 2016.