Mr. Sykes, who says he stands by much of his critique of the mainstream media, now concedes that years of relentlessly bashing it have left the country without any trusted sources of information.

“As a conservative talk show host, one of our themes has always been to bash the mainstream media, to point out the bias,” he said.

“But this is the moment you wake up and you go, O.K., have we done such an effective job that when I say, for example, ‘What Donald Trump said here is demonstrably false and here is the fact-check from The New York Times or The Washington Post’ — we now have an audience that is conditioned to say, ‘Well, you know, that’s the mainstream media, that is the liberal media, I’m not going to take that seriously.’ ”

The result, he said, is a country in which there are no final arbiters of truth.

“You begin to realize you’ve basically cut down all of these guideposts to be able to say, ‘Look this is what is reliable, this is what is not reliable,’ ” Mr. Sykes said.

Or as Mr. Rutenberg put it: in this new world, “Truth is absolutely relative. There’s no platonic form of truth.”