No candidate today is as daring as Herman Cain when it comes to charming an audience with silky silliness. His humor has distinguished him from his rivals, lending an air of folksiness and authenticity, a visceral contrast to his cooler, more staid rivals, especially Mitt Romney and President Obama.

Not all of Mr. Cain's attempts at comedy succeed, however. He tried to back away from what he called a joke about illegal immigration: that his policy would include an electrified border fence to kill anyone who tried to cross.

The moment illuminated one of the main concerns about Mr. Cain: He could have a hard time being taken seriously, at least to the degree that he uses sarcasm and laughs to divert attention from what for another candidate could be disqualifying gaps in knowledge and experience.

And while his casual style of racially inflected humor works to ingratiate him with mostly white audiences at campaign rallies, it has angered some black critics, who believe he uses age-old stereotypes.