Trump fired Comey last year over his frustration around the handing of the Russia investigation, which led to the appointment of the special counsel, Robert MuellerRobert Swan MuellerSasse: US should applaud choice of Mueller to lead Russia probe MORE.
Mueller's team is investigating, among other charges, the possibility that Trump's firing of his FBI chief was intended to obstruct the ongoing investigation into possible collusion between his campaign and Russia.
Comey went on to say there is a "danger" to democracy in America if voters don't respond to Trump's "threats to the rule of law".
Mr Comey told ABC's 20/20 programme on Sunday night: "I don't buy this stuff about him being mentally incompetent or early stages of dementia".
Comey said the Justice Department process worked well, but was "tainted" by Trump's public calls for McCabe to be fired. "We wake up in the morning and see the president of the United States is accusing people of crimes without evidence and pronouncing them guilty and saying they should be in jail".
"Our president must embody respect and adhere to the values that are at the core of this country".
Comey says Trump's actions "tainted an institution that pursues the truth".
A large man at 6-foot-8, Comey notes that he checked the president's hand size: It was "smaller than mine but did not seem unusually so".
On Monday, Trump pulled the plug on sanctions aimed at Russian companies behind the equipment related to Syrian President Bashar Assad's alleged chemical weapons attack, after the sanctions were announced Sunday by Nikki Haley, the U.S. ambassador to the United Nations.
STEVE INSKEEP, BYLINE: Why have you focused in some of your comments on what you view as the moral fitness of the president to be president?
"These are more words I never thought I'd utter about a president of the United States, but it's possible", Comey told ABC News' chief anchor George Stephanopoulos.
Former FBI director James Comey has said Donald Trump is "morally unfit to be president" and treats women like "pieces of meat".
Comey says Trump seemed fixated on an allegation made in a dossier compiled by a former British spy - and funded by Trump's political opponents - involving Russian prostitutes urinating on a bed in a Moscow hotel.
The president began assailing Comey even before the interview came out.
He seized on an excerpt shown Saturday in which Comey said his belief that Clinton would beat Trump in the 2016 presidential election was probably a factor in his decision to disclose the investigation into her emails.
"We're numb to it", he said. "People in this country need to stand up and go to the voting booth and vote their values". Ryan merely articulated what "reporters", and I use that term very loosely, nuance every day in those White House briefings. This is not OK.