Sen. Barack Obama, D-Ill., may have finally blown his chance at the Democratic nomination during Monday night's debate in South Carolina. For the first time, Obama got testy on national television -- and he didn't look good doing it.

The critical exchange occurred about 20 minutes into the debate. Sen. Hillary Clinton, D-N.Y., questioned Obama's stated admiration for Ronald Reagan. Then she attacked Obama's flip-flopping on the war in Iraq, stating that after he had opposed the war in 2002, he voted to fund it and removed an important anti-war speech from his website. Obama looked flustered; he stuttered; he moved his arms robotically; and finally he bust loose:

"While I was working on those streets watching those folks see their jobs shift overseas, you were a corporate lawyer sitting on the board at Wal-Mart," Obama almost shouted. Then he raised his voice even higher: "I was fighting these fights. I was fighting these fights."

Hillary quickly stifled a smile. She was smiling for a very good reason: Obama had finally entered the mud-wrestling pit. And the mud-wrestling pit is Hillary's territory.

Moments after Obama's response, he attacked Bill Clinton's involvement in Hillary's campaign. "I can't tell who I'm running against sometimes," he complained.

That was Hillary's cue to go on the offensive. "Well, you know, I think we both have very passionate and committed spouses who stand up for us. And I'm proud of that. But you also talked about the Republicans having ideas over the last 10 to 15 years. I was fighting against those ideas when you were practicing law and representing your contributor, Rezco, in his slum landlord business in inner city Chicago." The crowd oohed and aahed as Obama shook his head angrily.

As I explain in my new book, "Project President: Bad Hair and Botox on the Road to the White House," Barack Obama's appeal springs largely from his ability to rise above the fray. He speaks in broad generalities about hope and change, but rarely discusses hard policy. This is a calculated effort to create a winning image. Obama's obvious shortcomings are his youth and inexperience -- he minimizes those shortcomings by emphasizing his youth and avoiding the nitty-gritty political details at all costs.

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.