By Doina Chiacu and Angela Moore
WASHINGTON/NEW YORK (Reuters) - President Donald Trump attacked James Comey as a "weak and untruthful slime ball" on Friday after the fired former FBI director castigated him as an unethical liar and likened him to a mob moss in a searing new memoir.
The president fired Comey last May while his agency was investigating potential collusion between Trump's campaign and Russia in the 2016 U.S. election in a move that led the Justice Department to appoint Special Counsel Robert Mueller to take over a probe that has hung over his presidency.
"This president is unethical, and untethered to truth and institutional values," Comey said in the book due out Tuesday, a copy of which was obtained by Reuters.
Trump has often publicly criticized Comey since firing him, but escalated his attacks in response to the book.
"It was my great honor to fire James Comey!" Trump said in one of a series of scorching Twitter messages, adding that Comey - now one of the Republican president's fiercest critics - had been a terrible FBI director.
The tirade followed news accounts of Comey's book, "A Higher Loyalty: Truth, Lies and Leadership," which paints a deeply unflattering picture of Trump, comparing him to a mob boss who stresses personal loyalty over the law and has little regard for morality or truth.
Mueller is looking into whether Trump has sought to obstruct the Russia probe, and Comey could be a key witness on that front. Comey last year accused Trump of pressuring him to pledge loyalty and end a probe involving former national security adviser Michael Flynn's contacts with Moscow.
"James Comey is a proven LEAKER & LIAR," Trump wrote.
Trump accused Comey of lying to Congress, but did not specify was he was referring to, and said the former FBI chief should be prosecuted for leaking classified information.
Trump has denied any collusion and has called Mueller's investigation a witch hunt.
Comey is conducting a series of media interviews before the book's official release. Copies of the book were obtained by news outlets on Thursday.
The interviews are Comey's first public comments since he testified before the Senate Intelligence Committee last June, when he accused Trump of firing him to undermine the FBI's Russia investigation. Just days after Trump fired Comey, the president said he did it because of "this Russia thing."
Trump has launched a series of attacks since last year against U.S. law enforcement leaders and institutions as the Russia probe pressed forward, in addition to Comey and Mueller.
"People will rot in hell for besmirching the reputation the integrity and the professional history of these two men," Democratic U.S. Representative Jim Himes said on CNN, referring to Comey and Mueller, himself a former FBI director.
In an offshoot of the Mueller probe, Trump's longtime personal lawyer's office and home were raided by the Federal Bureau on Investigation on Monday.
In an interview broadcast on Friday on ABC's "Good Morning America," Comey discussed his initial encounters last year with Trump, who took office on Jan. 20, 2017.
He described Trump as volatile, defensive and concerned more about his own image than about whether Russia meddled in the presidential election.
American intelligence agencies last year said Russia interfered in the election through a campaign of propaganda and hacking in a scheme to sow discord in the United States and help get Trump elected. Moscow has denied meddling.
Comey said he cautioned Trump against ordering an investigation into a salacious intelligence dossier alleging an 2013 encounter involving prostitutes in Moscow.
The dossier was compiled by former British intelligence officer Christopher Steele about Trump's ties to Russia and included an allegation that involved prostitutes urinating on one another in a hotel room while Trump watched.
Trump denied the allegations and said he might want the FBI to investigate allegations in the dossier to prove they were untrue, Comey told ABC.
"I said to him, 'Sir that's up to you but you want to be careful about that because it might create a narrative that we're investigating you personally and, second, it's very difficult to prove something didn't happen,'" Comey said.
Asked to describe that Jan. 6, 2017 meeting two weeks before Trump took office, Comey said: "Really weird. It was almost an out-of-body experience for me."
Comey was asked if he believed the dossier's allegations.
"I honestly never thought these words would come out of my mouth, but I don't know whether the current president of the United States was with prostitutes peeing on each other in Moscow in 2013," Comey told ABC. "It's possible, but I don't know."
Comey said the dossier's allegations had not been verified by the time he left the FBI.
Before Trump and Comey met alone, U.S. intelligence chiefs briefed Trump and his advisers about the Russian election meddling. What struck him most, Comey told ABC, was that the conversation moved straight into a public relations mode, what they could say and how they could position Trump.
"No one, to my recollection, asked, 'So what's coming next from the Russians, how might we stop it, what's the future look like?'" Comey said.
(GRAPHIC: Major milestones in the Mueller probe - https://tmsnrt.rs/2GTgtnX)
(Reporting by Doina Chiacu in Washington and Angela Moore in New York; Additional reporting by Justin Mitchell in Washington; Editing by Frances Kerry and Will Dunham)