Comey's comments in an ABC News interview that aired Sunday were nearly certain to escalate his war of words with the president and further erode a relationship marked by open hostility and name-calling.
Comey's claim that unverified classified material exists that could have "cast serious doubt" on then-Attorney General Loretta Lynch's "independence" with the Clinton investigation.
On May 16, The New York Times published an article about a memo Comey reportedly wrote that detailed a February 14 meeting between Comey and Trump.
"I mean, it's certainly some evidence of obstruction of justice", he said.
The president said Comey "stupidly" handled a probe into Trump's 2016 election rival Hillary Clinton and how she used a private email server during her time as secretary of state. Comey also revealed during the hearing that he documented his interactions with Trump because he was "concerned" that the President "might lie about the nature of our meeting".
He says he knows that's a "stunning" possibility and something that he never thought he'd say about a president of the United States.
I don't think he's medically unfit to be President. "But instead, what I'm talking about is that leadership culture constantly comes back to me when I think about my experience with the Trump administration". However, he conceded that he "should've worked harder to find a way to convey that it's more than just the ordinary mistake, but it's not criminal behavior".
Comey: Found no indication that it was true.
Parts of the interview are likely to revive the fury of Clinton supporters who think he cost her the presidency by reopening the email investigation less than two weeks before the election. It was the passage he struggled most to read when he recorded the audio edition of his book, A Higher Loyalty: Truth, Lies, and Leadership, which will be published Tuesday by Flatiron.
He calls Trump "untethered to the truth" is "ego-driven" and runs the White House like a "mob boss".
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.
Despite his misgivings, Comey said he would not be in favor of impeaching Trump.
"It must have been", Comey told Stephanopoulos.
He replied: "I don't remember spelling it out, but it had to have been". And I'm sure a whole lotta people have a view of me based on that. I have many (too many!) lawyers and they are probably wondering when their offices, and even homes, are going to be raided with everything, including their phones and computers, taken.
Comey's own firing in May 2017 by Trump set in motion the appointment of special counsel Robert Mueller by the Justice Department.
Mueller's probe has expanded to include whether Trump obstructed justice by firing Comey. So far, 19 people have been charged in the investigation.
This morning, The Atlantic's Natasha Bertrand pointed out a discrepancy between Comey's recollection of his conversation with Trump and reported testimony in front of the House Intelligence Committee regarding that infamous night in Moscow.
"I'm sorry that I caused all kinds of confusion and pain with the way I described her conduct that led people into all kinds of side roads", Comey said.