Comey, who ran the Federal Bureau of Investigation for nearly four years, portrays the president in the book as a liar, dumb and rude, and he compares elements of Trump's behavior to that of a Mafia boss.
'I don't know whether any of this true, but this is what he said, ' said Comey.
'These are more words I never thought I'd utter about a president of the United States, but it's possible, ' Mr Comey said. He said he hoped that people would see "I'm not trying to help a candidate or hurt a candidate; I'm trying to do the right thing".
Comey added that he thinks its "possible" Russian Federation has compromising material on Trump, albeit "unlikely". But if Trump did indeed sleep over at the Moscow Ritz-Carlton, what, exactly, would be motivating him to say that he didn't? And so I don't know the reason.
Trump fired Comey in May 2017, citing the FBI's behavior in investigating Democrat Hillary Clinton and its probe into possible Trump campaign collusion with Russian Federation to tilt the 2016 presidential election.
U.S. President Donald Trump departs after giving remarks on tax cuts for American workers during an event in the White House Rose Garden in Washington, U.S., April 12, 2018.
"Possibly. I mean, it's certainly some evidence of obstruction of justice", Mr Comey said.
But he said in Trump's case, he could not discount the possibility that the president had been compromised. But, he acknowledged, "I was operating in a world where Hillary Clinton was going to beat Donald Trump, and so I'm sure that it was a factor". Comey agreed the alleged actions were in line with those of a mob boss, and said he doesn't make that claim lightly. Comey said he urged Kelly to remain in the administration.
'You can't say for certain that the president of the United States is not compromised by the Russians?' pressed Stephanopoulos. "But in many ways, I feel great pressure to stay to try and protect the institution I lead".
However he insisted he would have still sent the letter to congress which announced the FBI was re-opening investigations into Mrs Clinton's email, just days before the election, even if he knew the letter would elect Mr Trump. In a statement Sunday, Lynch said she was simply following longstanding Justice Department protocol against confirming or denying the existence of an investigation. Instead, he writes, they launched into a strategy session about how to "spin what we'd just told them" for the public.
Comey, however, described in his memoir that his concern lay not with keeping his job, but making Clinton appear to be an "illegitimate president" by not disclosing the fact that the FBI had restarted its investigation into her use of a private email server.
The special counsel Robert Mueller is now investigating the Trump campaign - and Trump himself - as part of the Russian Federation probe.
Trump cited that meeting in the same tweet, writing, "Why can't we all find out what happened on the tarmac in the back of the plane with Wild Bill and Lynch?" Framing his choice as between "speak" and "conceal", he argued that if it only came out after the election that the FBI had reopened its investigation into Clinton's emails, that would "destroy the institutions that I love".