Franken gets press and attention thanks to O'Reilly. Before the book expo and the lawsuit, I don't remember him getting much press since the release of "Rush Limbaugh Is A Big Fat Liar."
I guess he's a political humorist. He's a liberal comedy writer who has taken his convictions and his gift for writing and used them to become what he sees as an aggressive voice for the left. He's upset that people like Coulter, Savage, and O'Reilly accuse the left of "resorting to insults," lying, etc., when they're down in the mud with the worst of them. But as I said, I think he spoils his arguments with his childish name-calling. He seems to take everything very personally.
As for why any of the candidates care about him, I couldn't tell you. Maybe they're out of touch enough to believe that any best-selling author lends their campaigns instant credibility...
