David Letterman annihilates Trump: He’s 'a damaged human being — a person to be shunned'

David Letterman knows Donald Trump pretty well after interviewing him frequently over the years on his television show -- but even he has been surprised by the Republican nominee's outrageous behavior on the campaign trail.

The former late night talk show host said he loved Trump as a television guest, because his "blowhard billionaire" persona was instantly recognizable to audiences, reported the New York Times.

"Nobody took him seriously, and people loved him when he would come on the show," said Letterman, who retired last year. "I would make fun of his hair, I would call him a slumlord, I would make fun of his ties -- and he could just take a punch like nothing. He was the perfect guest."

Letterman said he's surprised that Trump's presidential campaign has been so successful, but there's one moment that he thinks should have disqualified the real estate developer and former reality TV star.

"I can remember him doing an impression, behind a podium, of a reporter for the New York Times who has a congenital disorder, and then I thought, if this was somebody else — if this was a member of your family or a next-door neighbor, a guy at work — you would immediately distance yourself from that person," Letterman said. "And that’s what I thought would happen. Because if you can do that in a national forum, that says to me that you are a damaged human being. If you can do that, and not apologize, you’re a person to be shunned."

He was sharply critical of "Tonight Show" host Jimmy Fallon, who earned scorn and mockery for his softball interview of Trump last month.

"If I had a show, I would have gone right after him," Letterman said. "I would have said something like, 'Hey, nice to see you. Now, let me ask you: What gives you the right to make fun of a human who is less fortunate, physically, than you are?'"

Letterman said that should really have been enough to sink Trump's campaign.

"I don’t know anything about politics," Letterman said. "I don’t know anything about trade agreements. I don’t know anything about China devaluing the yuan. But if you see somebody who’s not behaving like any other human you’ve known, that means something. They need an appointment with a psychiatrist. They need a diagnosis and they need a prescription."

Watch Letterman confront Trump in 2012 over moving his clothing manufacturing operation to China: