Former FBI director James Comey has said Donald Trump is “morally unfit to be president”, alleging he treats women like “pieces of meat”.
Mr Comey was giving his first major television interview since he was fired by President Trump last year.
He told ABC News that Mr Trump lies constantly and may have obstructed justice.
Hours before the interview aired, the president went on the offensive, accusing Mr Comey of “many lies”.
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.”
“I don’t think he’s medically unfit to be president. I think he’s morally unfit to be president.
“Our president must embody respect and adhere to the values that are at the core of this country. The most important being truth. This president is not able to do that,” Mr Comey said.
After the interview aired, Mr Trump’s party – via the Republican National Committee – released a statement saying Mr Comey’s publicity tour for his new book showed “his true higher loyalty is to himself”.
“The only thing worse than Comey’s history of misconduct is his willingness to say anything to sell books,” it said.
It is the latest development in a long-standing feud between the two men, further fuelled by the upcoming publication of Mr Comey’s memoir A Higher Loyalty: Truth, Lies and Leadership.
The ex-FBI chief is on a publicity blitz for the book.
President Trump has said the” badly reviewed book” raises “big questions”.
He also suggested Mr Comey should be imprisoned, and in recent days began referring to him as a “slimeball”.
The story dates back to the 2016 presidential election, when Mr Comey was FBI director, and the investigation into Democrat candidate Hillary Clinton’s handling of classified emails on a private server while Secretary of State.