John Dickerson scrutinizes the GOP candidates' senses of humor:
A joke well-told gives the audience something they can pass along later to their friends. It magnifies your message easily or at least makes voters feel good enough that they report back favorably about their experience at your rally. Harry Truman's 1948 campaign—and arguably his presidency—was defined in a joke. During a speech attacking Republicans during that campaign, a supporter yelled "Give 'em Hell, Harry!" Truman replied, "I don't give them Hell. I just tell the truth about them and they think it's Hell." That quip became Truman's nickname.
The Dish recently ran some cringe-worthy jokes from the candidates here, here, here, here and here.
The Dish 