I don't really know that he did that much wrong. He made a pretty bad gaffe in the summer, I guess. But otherwise his campaign has been fine. Nothing inspiring obviously, but not godawful. The problem has been that the media's been in the tank for Ernst because she's photogenic and has a compelling personal story, and so they've been eager to attack Braley over the dumbest non-stories but completely unwilling to actually discuss Ernst's extremist positions.
Yes, its a paradox that someone as radical as Ernst (probably) gets elected in a state as moderate as Iowa.