Donald Trump’s “emotional maturity [and] stability” are being discussed in private by senior members of his own political party, according to veteran Washington journalist Carl Bernstein, in a turn of events he has described as unprecedented.

Bernstein was speaking on CNN late on Wednesday night, when he revealed there was “open discussion” among Republicans about Mr Trump’s fitness to rule.

Mr Trump has continued to repeat unfounded claims that between three and five million illegal votes were cast in the 2016 election, helping sway the result of the popular vote against him. There is no evidence to support such allegations, and they have been met with dismay by senior Republicans.

Bernstein said discussions going on in Washington this week were “unlike anything I have seen in 50 years as a reporter”.

“I am hearing from Republicans, and other reporters are as well, that there is open discussion by members of the President of the United States’ own party about his emotional maturity, stability,” he said.

“People are saying that his psyche is driving the news cycle.

“We are in uncharted territory here and we ought to talk to some of our colleagues about what they are hearing. I’ve never heard [people] talking about a president… the way this subtext is now a talking point.”

Bernstein is one of the reporters who broke the infamous Watergate scandal, and now serves as a regular political commentator on CNN.

In an appearance on a CNN panel earlier in the week, Bernstein clashed with Trump supporter Kayleigh McEnany, who claimed “there is no way to disprove” the President’s belief about illegal voting.

"We are having too many bizarre moments with this new President,” he said. “You want to say to the President of the United States: Grow up, Mr President.