President Donald Trump is such an indiscriminate liar that he sometimes doesn't even know he's lying, former FBI Director James Comey said in an interview with ABC News that aired Sunday night. He said Trump was "morally unfit" to hold his office.

In hours-long interview with ABC's George Stephanopoulos, parts of which aired on "20/20," Comey recounted many of the details he revealed in his memoir, "A Higher Loyalty: Truth, Lies and Leadership." The interview was conducted before NBC News and other news organizations obtained copies of the book and reported many of its details last week, so the discussion included no responses to Trump's tweeted outrage since then.

Slippery James Comey, a man who always ends up badly and out of whack (he is not smart!), will go down as the WORST FBI Director in history, by far! — Donald J. Trump (@realDonaldTrump) April 15, 2018

"Our president must embody respect and adhere to the values that are at the core of this country, the most important being truth," Comey said last week, according to a transcript of the entire interview that ABC News published Sunday night. "This president is not able to do that. He is morally unfit to be president."

Trump lies are so promiscuous that "sometimes he's lying in ways that are obvious, sometimes he's saying things that we may not know are true or false, and then there's a spectrum in between," Comey said, according to ABC News.

"The challenge of this president is that he will stain everyone around him," he said.

And he indicated that the president's behavior in a one-on-one meeting could have been criminal.

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Recounting a key section of his book, Comey said that in a meeting in February 2017, Trump asked Comey to drop the FBI's investigation of Michael Flynn, who had recently resigned as his national security adviser.

He said that when Trump asked him privately to drop the investigation of Flynn — an allegation Trump has denied — it was "certainly some evidence of obstruction of justice." But he said any criminal charge would be up to a prosecutor, who would evaluate Trump's intention.

White House press secretary Sarah Huckabee Sanders told reporters during a briefing aboard Air Force One on the way to Florida on Monday that the president did not watch the entire Comey interview, but saw "bits and pieces" and didn't "learn anything new." She also labeled him a "self-admitted leaker" who has "proven to be dishonest."

Comey revealed little in the interview that isn't in the book. But one thing he did offer was his confirmation that the FBI already knew all of the major allegations in the so-called Steele dossier before it even learned of the much-debated 35-page opposition-research document alleging that Trump's presidential campaign colluded with Russia.

While much of the president's invective about the investigation — conducted first by the FBI and then by special counsel Robert Mueller — centers on the opposition dossier, Comey told ABC News that "the investigation was triggered entirely separately from the Steele dossier."

The investigation actually began when the FBI learned in July 2016 that George Papadopoulos, who was briefly an adviser to the Trump presidential campaign, "had been talking to someone in London about getting dirt that the Russians had on Hillary Clinton."

"The FBI didn't get any information that's part of the so-called Steele dossier, as I understand it, until after that," he said.

Regardless, he said, once it learned of the dossier, the FBI considered Christopher Steele, the former British intelligence officer who compiled it, to be a credible source, calling him "someone with a track record, someone who was a credible and respected member of an allied intelligence service during his career."

Comey also spoke at length about what he characterized as his "no-win situation" in his deliberations over whether to announce publicly, less than two weeks before the presidential election, that the FBI was looking into more emails connected to its investigation of Democratic nominee Hillary Clinton's use of a private email server when she was secretary of state.

Comey wrote in "A Higher Loyalty" that he assumed Clinton would win — as virtually every major poll suggested — and that he didn't want to tarnish Clinton's presidency as illegitimate if it were to be learned later that the FBI had withheld the information.

Comey told ABC News that he would make the same decision again.

"If I ever start considering whose political fortunes will be affected by a decision, we're done," he said, according to ABC News. "We're no longer that group in America that is apart from the partisans and that can be trusted. We're just another player in the tribal battle."

James Comey joins Rachel Maddow live in-studio Thursday at 9 p.m., ET, on MSNBC.

CORRECTION (April 16, 2018, 6:10 a.m.): An earlier version of this article misstated James Comey’s title. He is a former director of the FBI, not the current director.