Former FBI Director James Comey says he thinks Donald Trump is morally unfit to be the US president.
In an interview with ABC News that aired on Sunday night, Mr Comey also said he thought it was possible the Russians had compromising information on Mr Trump, and that there was "some evidence of obstruction of justice" in the president's actions.
His comments were almost certain to escalate his war of words with the president and further erode a relationship marked by open hostility and name-calling.
Hours before the interview aired, the president, who fired Mr Comey last year, unleashed a Twitter outburst that labelled Mr Comey "slippery", suggested he should be put in jail and branded him "the WORST FBI Director in history, by far!"
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
Mr Comey's televised remarks, coupled with the release of his forthcoming book, offer his version of events surrounding his firing and the investigations into Russian election meddling and Hillary Clinton's email practices.
Several of the episodes he describes in detail, including a private conversation about former White House national security adviser Michael Flynn, are central to special counsel Robert Mueller's investigation and his recollections are presumably valuable for prosecutors examining whether the president's actions constitute obstruction of justice.
The FBI director, who until his firing last May led an investigation into possible ties between Russia and the Trump campaign, acknowledged that it was "stunning" to think that Russia could have damaging information about an American president. But he said that in Mr Trump's case, he could not discount the possibility that the president had been compromised.
"These are more words I never thought I'd utter about a president of the United States, but it's possible," Mr Comey told ABC News' chief anchor George Stephanopoulos.
He also answered "possibly" when asked if the president was attempting to obstruct justice when he cleared the Oval Office of other officials last February before encouraging him to close the investigation into Mr Flynn, who by that point was suspected of lying to the FBI about his Russian contacts.
The retired general pleaded guilty last December and is now cooperating with Mueller's investigation.
Mr Comey also said he believed that Trump was "morally unfit" to be president and that he treated women like "pieces of meat".
"A person who sees moral equivalence in Charlottesville, who talks about and treats women like they're pieces of meat, who lies constantly about matters big and small and insists the American people believe it - that person's not fit to be president of the United States, on moral grounds," he said.
On Sunday Mr Trump rejected Mr Comey's assertion that the president had sought his loyalty at a January 2017 dinner, saying "I hardly even knew this guy. Just another of his many lies".
Mr Trump fired Mr Comey in May 2017, setting off a scramble at the Justice Department that led to the appointment of Mueller as special counsel overseeing the Russia investigation.
Mr Mueller's probe has expanded to include whether Mr Trump obstructed justice by firing Mr Comey. So far, 19 people have been charged in the investigation.
Asked whether he believed Mr Trump ought to be impeached, Mr Comey replied: "I hope not because I think impeaching and removing Donald Trump from office would let the American people off the hook and have something happen indirectly that I believe they're duty bound to do directly. People in this country need to stand up and go to the voting booth and vote their values."
He added: "But you cannot have, as president of the United States, someone who does not reflect the values that I believe Republicans treasure and Democrats treasure and Independents treasure. That is the core of this country. That's our foundation. And so impeachment, in a way, would short-circuit that."