Do Presidential candidates have some sort of legal immunity? ?

When a Presidential candidate makes television adds basically saying that they think that the other candidate is an idiot, what stops the other candidate from being able to sue. I mean, if "Joe Schmo" says that he thinks his neighbor is an idiot, then I think the neighbor could sue for defamation of character, or something. So, to restate my question: do Presidential candidates have some sort of legal immunity?