Mr Comey was asked if the President had obstructed justice by directing him to drop the investigation into Michael Flynn, the former national security adviser.
Throughout the interview, Comey stressed the importance of telling the truth, a theme in his book. "This president is not able to do that".
In an excerpt shown on Saturday, Mr Comey said his belief Mrs Clinton would beat Mr Trump in the 2016 presidential election was a factor in his decision to disclose the investigation into her emails.
Comey says Steele's track record was why he felt it "was important that we try to understand [the dossier], and see what could we verify, what could we rule in or rule out".
The comments were made in Comey's first interview since writing his book, "A Higher Loyalty: Truth, Lies and Leadership", which is scheduled for release Tuesday.
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.
But when it rains.it pours.at least some of the media seem to be pouring it on the president right now with the latest cover of Time Magazine showing Trump under 'stormy conditions'.
The hope, says CNN writer Josh Campbell, is that discrediting Comey will discredit the FBI as a whole, in essence weakening the investigation of special counsel Robert Mueller into suspected collusion by the Trump campaign and Russian operatives to interfere with the 2016 election.
"I think it's possible". I don't think he's medically unfit to be president.
Gergen said Comey's book could "change the narrative" about the Trump presidency.
"Values matter. This president does not reflect the values of this country", he said of Donald Trump. "He is morally unfit to be President". Comey added, "I don't know whether the current President of the United States was with prostitutes peeing on each other in Moscow in 2013".
He added that U.S. citizens need to "fight" for American values, and that "impeachment, in a way, would short-circuit that". Trump has called the special counsel probe a "witch hunt" and repeatedly insisted there was no collusion between his campaign and Russian Federation.
Comey acknowledged in the ABC interview that his belief that Clinton would be elected president "was a factor" in his decision to reopen the email probe 11 days before the USA election, a development that Clinton blames for her surprise defeat. But Trump later told NBC's Lester Holt that "this Russian Federation thing" had been a factor in his decision.
The former FBI Director said that he was "actually nervous" to meet Trump in person.
"Virtually everyone in Washington thought he should be fired for the bad job he did-until he was, in fact, fired", Trump tweeted Friday. In other words, he was making decisions based on the fact that he thought she was going to win, and he wanted a job.
Comey said that struck him as odd. Democrats have faulted Comey for his handling of the investigation into Hillary Clinton's use of a private email server as secretary of state. That's not a perspective, that's not a view, that's just a lie.
Comey said the allegations in the dossier had not been verified at the time he left the Federal Bureau of Investigation.