Journalism heavyweight Carl Bernstein called on the media to rise to the challenges of covering Trump's presidency, which he described as "malignant".
"We're in foreign territory," Bernstein said on CNN's New Day. "We have never been in a malignant presidency like this before. It calls on our leaders, it calls on our journalists to do a different kind of reporting, a different kind of dealing with this presidency and the president."
CNN has been a particular target of the president's, with Trump posting a video of himself body slamming a CNN logo on Sunday (2 July). The president has also taken to Twitter to call CNN and the media fake news and fraud news.
Bernstein added that Trump's presidency was beginning to worry leaders in Washington.
"I think something much greater is happening, and that is that we are in the midst of a malignant presidency," he said. "That malignancy is known to the military leaders of the country. It's known to the Republican leadership in Congress who recognise it, and it's known to the intelligence community."
He continued: "That has got our leaders worried. They are worried about his character. They are worried about his capabilities. They are worried about his temperament and the state of his temperament, to use kind words here."
Bernstein, whose reporting on the Watergate scandal for The Washington Post led to President Richard Nixon's resignation in 1974, said reporters should talk to leaders in Washington about their confidence in Trump's ability to lead the country.
"Many members of Congress, many of our military leaders, many in our intelligence community who I've spoken to, and I know other reporters have spoken to, understand and are beginning to comprehend that the underlying story here is a lack of confidence in the abilities and character of the president of the United States of America," he said.
"This president is not in control of the presidency in a way that it is functioning, and that has got our leaders worried," he continued. "We have to...be kind of medical reporters right now. I don't mean about the president's psyche, but rather about every aspect of his presidency, and how and whether it is functioning, because many aspects are not functioning."
Bernstein's remarks come as the president vehemently defended his attacks on the media and his constant tweeting.
"The FAKE & FRAUDULENT NEWS MEDIA is working hard to convince Republicans and others I should not use social media- but remember, I won," he tweeted on Saturday (1 July). "....the 2016 election with interviews, speeches and social media. I had to beat #FakeNews, and did. We will continue to WIN!"
"My use of social media is not Presidential—it's MODERN DAY PRESIDENTIAL. Make America Great Again!" he added.