Most of all, though, Obama's image is an optimistic one. He's the only Democratic candidate who smiles on a consistent basis. He's energetic and charismatic -- and he's African-American, which only enhances his freshness.

What Obama cannot afford -- and what he did in South Carolina -- is to look angry or mean. By attacking Hillary Clinton, Obama looked angry and mean. He seemed petulant in New Hampshire when he sneered that Hillary was "likable enough." He seemed unpleasant in South Carolina when he blasted Hillary's legal career. And he seemed downright nasty when he blasted Bill's involvement in her campaign.

Hillary Clinton can afford to look mean on occasion -- the American public is used to it. And Hillary is keeping that meanness to a minimum by deploying Bill as her campaign attack dog.

Obama, by contrast, cannot afford to look mean. The moment the smile comes off his face, Obama becomes just another negative candidate, overshadowed by Clinton star-power and political prowess.

"We're just getting warmed up," Hillary chuckled as Obama's ire began to rise. Hillary may be getting warmed up, but as Obama's temper heats up, his campaign cools down.