Former FBI director James Comey has said Donald Trump is "morally unfit to be president", who 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.