Al Franken earned a national following long before he entered politics. His work in the original cast of NBC's "Saturday Night Live" began a long career as a comedian. He grew more politically active over time as a left-leaning author and radio host before launching an improbable Democratic bid for the U.S. Senate from Minnesota in 2008. Franken almost lost in a hail of GOP attacks that used off-color bits from his comedic career to cast doubt on his values. After triumphing in a lengthy recount, he purposely submerged his wit while building a reputation for seriousness during his first term. It worked well enough that he won re-election easily in 2014. Earlier this year, Franken won applause from Democrats nationally for challenging the new Trump administration, including with sharp questioning at the confirmation hearing of Attorney General Jeff Sessions.

His humor-laced new book, "Giant of the Senate," marks a shift in his approach — and the beginning of speculation that Franken may enter the 2020 presidential race. The 66-year-old senator sat down with CNBC Editor at Large John Harwood at the Minnesota State Fair to discuss his book, his future and Trump. What follows is a condensed, edited transcript of their conversation.

Senator Al Franken sits down with CNBC’s John Harwood at the Minnesota State Fair to talk politics and comedy on Aug. 31st, 2017. Sophie Bearman | CNBC

Harwood: You say in your book that lying is something that bugged the crap out of you. Franken: Yeah. Harwood: It makes politics a weird career to go into. Franken: No, not really. I mean, I think it helps not to lie in politics. Harwood: What's the evidence for that? Franken: Well, we don't have the evidence right now on terms of the president. Harwood: You allude in the book a couple of times to the — to the idea that there may be something wrong with him. Franken: I'm not a psychiatrist. And even a psychiatrist needs to examine someone in order to make a diagnosis. I'm not qualified to do that. But I do think that his actions have been well outside the norm for a president and, in many ways, for a human being. Harwood: You have friends in the Republican caucus. What do they say? Franken: I don't think there's any doubt among my colleagues that he has some temperament issues, that he does seem to see everything in terms of how it affects Donald Trump. That he is almost pathological in the way he lies.

Harwood: Do you expect Trump to serve four years? Franken: I don't know. I think we have the right guy as special counsel. I think Bob Mueller will get to the bottom of this. I think he'll just go where the facts take him. That's what we want him to do. Harwood: The issues that caused you some turbulence in the 2008 campaign — the Playboy article, all that stuff that you write about in the book — has all that been washed away by time? Is all that irrelevant now? Franken: Yeah, I think so. Harwood: Does Trump stand for the proposition that none of this stuff matters anymore anyway? Franken: Well, yeah. I mean, I write a lot about my first race and what that was like. And to go back and look at that … Harwood: It sort of looks like the 19th century now. Franken: He didn't bow correctly when the lady entered the room. Harwood: You can lay the Lesley Stahl 2 a.m. joke writing session next to the "Access Hollywood" tape. Franken: Yeah, I would be happy to do that.

Harwood: So, in some way, the voters have adjusted to you, Trump, and other politicians as political fashion changes. Franken: Wait a minute. You lumped me in with Trump? Take that back. Take that back, John Harwood. You're not taking it back. Harwood: I'm just saying voters get conditioned to what they see in front of them in politics. Franken: Well, no. One of the reasons that I comfortably won the second time is they saw that I worked very hard, paid attention to them, to Minnesotans, and paid attention to policy and did my work. Donald Trump is the opposite. He doesn't work. He doesn't know how policy works. He won't do his homework. He refuses to work on anything that matters. So, I consider myself a polar opposite of him. I mean, I really do. And the fact that we are both in a branch of show business — he was in reality TV. A human cannonball was in show business, you know? A rodeo clown's in show business. Harwood: I know you are not planning to run for president. But other people are talking about it. How do you think about that issue? Franken: Well, people have brought it up, but I think they bring it up about a lot of people. I think that the president of the United States should be someone who really wants to be president of the United States. I've seen the job a little bit more up close as a senator than I did as a comedian. I can see what an incredibly high-pressured job it is. And it's not something that I've aspired to ever. Harwood: But you're not saying straight-up there is zero chance you're going to do that. Franken: There's pretty much a zero chance, I think, I mean, unless, you know, I get hit in the head. Harwood: Your colleague, Sen. [Amy] Klobuchar, is in the same position. Do you talk to her about this? Franken: I feel like this is something I don't want to — I just don't broach with her. Harwood: We've got a celebrity-driven culture. And people are very used to back and forth between politicians. So I wonder if you think that humor provides a different dimension that would be valuable either for you or for somebody else running for president? Franken: Yeah. I think a sense of humor is great in life. You know, I'm funny. And I've bonded with all my colleagues. Harwood: So, it's valuable. Franken: Yeah, it's valuable. Your life would be better if you got a sense of humor, John. Harwood: I'm working on it. I concluded from your book that you remain a little bit irritated with Obama and the distance that he kept from you in 2008. Franken: Not really. I know it might sound like that. I was a little peeved because I clearly was the closest race that year in 2008. But I can understand better now why he didn't. Harwood: Even after the election when you were trying to raise money for your recount? Franken: That, I still — I don't know who to fault for that, him or his team. Harwood: Have you talked to the president about it? Franken: I have not. And I hope to. I hope to because I know he'll probably see it in the book. I had to raise, like, $13 million for the recount. People might say, "What?" Well, it was lawyers. And he was, like, high-60s [approval rating]. All he had to do was put on a tux and, boom, $3 million, you know? That would be a lot of call time. By the way, I think he was a great president. Harwood: When you were doing [Sen.] Paul Simon [on "Saturday Night Live"], did you want to be Paul Simon? Franken: No. I did not want to run for the Senate, really, until Norm Coleman said he was a 99 percent improvement over Paul Wellstone.