Former talk show host David Letterman has tagged US President Donald Trump as "crazy", saying he would feel "exhausted" if he had to interview him.

"If I still had a show, people would have to come and take me off the stage. 'Dave, that's enough about Trump. We've run out of tape.' It's all I'd be talking about. I'd be exhausted," Letterman told media.

When asked how the interview would go, Letterman said he would not begin the conversation by listing off his grievances; but instead would go right into a lecture. 

"I think I would be in the position to give him a bit of a scolding and he would have to sit there and take it. Yeah, I would like an hour with Donald Trump; an hour and a half," he said.

In terms of dealing with Trump now, Letterman said citizens "need to figure out ways to protect ourselves from him. We know he's crazy. We gotta take care of ourselves here."