CNN host Brian Stelter has faced criticism for revealing that his colleagues in the mainstream media regularly discuss whether President Trump is mentally fit to hold office.

During Sunday's edition of "Reliable Sources," Stelter said such conversations were happening "in newsrooms and TV studios" in the wake of Trump's response to the previous weekend's violence between white nationalists and counter-protesters and Charlottesville, Va.

"Usually after the microphones are off, or after the stories are filed, after the paper has been put to bed, people’s concerns, and fears and questions come out," Stelter said. "... Questions like these: Is the president of the United States a racist? Is he suffering from some kind of illness? Is he fit for office? And if he’s unfit, then what?"

Stelter's segment was met with a furious response from social media users, including Fox News' Geraldo Rivera.

Stelter answered Rivera that he had never called Trump a "mental case" and doubled down on his original claim, saying "some journos and many other folks are worried about Trump's fitness. And that's a fact."

Undeterred, Rivera said Stelter had inadvertently revealed the network's "rotten bias" by going "a bridge too far."

Led by Stelter, a former New York Times reporter, "Reliable Sources" has emerged as a determined anti-Trump presence both over the airwaves and on social media. Former Washington Post reporter Carl Bernstein, a frequent guest, has used the program as a platform to compare Trump unfavorably to Richard Nixon. On at least one occasion last year, the former Watergate reporter described Trump as a "neo-fascist sociopath."

Another frequent guest, liberal historian Douglas Brinkley, said Trump was "a sick man in the White House."

"Look, we all know [Trump] is a neon billboard for, you know, overt narcissism, malignant self-love. We’ve all known that," Brinkley said, later adding, "He's not mentally stable."