Mr. Comey will have an opportunity to respond during what is expected to be a long book tour in which he will do many interviews. His first major interview, with ABC News, is scheduled to be broadcast Sunday night, though the network began airing clips Friday morning after the book leaked out.
In the meantime, Mr. Comey is likely to receive a somewhat muted defense from some Democrats, who are still angry about the way he handled the investigation into Hillary Clinton’s email server. While they cheer on his fight with Mr. Trump, they argue Mr. Comey should not have made public the email probe the way he did.
“He let his own ego get in the way, and it put him in charge of fate that was not his decision to act on,” said Jennifer Palmieri, a senior adviser to Mrs. Clinton’s campaign. “I don’t think he had partisan motivations. But there’s a lot of people I know who don’t agree with me on that.”
Anger toward Mr. Comey from Democrats cascaded across social media sites on Friday. Ms. Palmieri said she would urge Democrats not to join Mr. Trump in piling on Mr. Comey, even though she admitted there is “a lot of resentment” toward him.
“I don’t agree that he’s an untruthful slimeball,” she said, adding that Democrats should not help the president undermine Mr. Comey’s credibility. “That’s not responsible or productive.”
Nevertheless, the insult slinging has officially begun, and neither Mr. Trump nor Mr. Comey minced their words.
Mr. Trump’s face appeared “slightly orange, with bright white half-moons under his eyes where I assumed he placed small tanning goggles,” Mr. Comey writes.
Mr. Comey is “weak,” and was a “terrible Director,” Mr. Trump said.
Mr. Trump fired Mr. Comey in May of last year, a decision that eventually led to the appointment of a special counsel to investigate Russia’s 2016 election meddling and whether Mr. Trump has deliberately tried to obstruct the investigation.
The former F.B.I. chief’s much-anticipated 304-page memoir is scheduled to be released next week. It is the first major memoir by one of the key characters in the Trump administration.
In his book, Mr. Comey reflects on his days as a mob prosecutor in New York and compares Mr. Trump to a mafia boss: “The silent circle of assent. The boss in complete control. The loyalty oaths. The us-versus-them worldview. The lying about all things, large and small, in service to some code of loyalty that put the organization above morality and above the truth.”
Mr. Comey also describes a January 2017 dinner where he said Mr. Trump asked him for a loyalty pledge, an episode that was reported by multiple news organizations last year.
Republican allies of Mr. Trump’s have planned a counteroffensive and created a “Lyin’ Comey” website aimed at discrediting the former F.B.I. chief.
And Fox News, the president’s preferred TV news network, plans to air a special called “The Trial of James Comey” Sunday night. The Fox program is scheduled to air on The Next Revolution with Steve Hilton at 9 p.m.
The White House press secretary, Sarah Huckabee Sanders, tweeted early Friday that Mr. Comey “has no credibility.”
Kellyanne Conway, one of Mr. Trump’s advisers, hit on the same theme.
“We find that Mr. Comey has a revisionist view of history and seems like a disgruntled ex-employee,” Ms. Conway said on Friday. “After all, he was fired.”