She can't win. The pundits say it. The polls say it. The math says it. It's even the word on the street. If Huggy Bear from "Starsky and Hutch" were around, he'd say it's time to stick a fork in her.

So why does she keep going?

One theory is psychological, almost Aesopian. Hillary Clinton - like her husband - is a creature who follows her nature. Scorpions must sting. Ants must save food for winter. Clintons must fight.

Bill Clinton illustrated Clinton grit when he confronted Newt Gingrich during the government shutdown of the mid-'90s. "Do you know who I am?" Clinton said to Gingrich. "I'm the big rubber clown doll you had as a kid, and every time you hit it, it bounces back. That's me - the harder you hit me, the faster I come back up."

"That was not bravado," writes Commentary's John Podhoretz, author of a book about Hillary Clinton titled "Can She Be Stopped?" "It was a warning, and an accurate one. The Clintons are without shame, and therefore we all believe they are without honor and cannot possibly imagine themselves as heroes. But Bill very much believed, and believes, that he is a hero because he would not allow himself to be defeated, no matter what - and that part of his eventual victory would be that he could use the virulence of his foes to his advantage."

Hillary sees herself the same way. The Clintons have campaigned as a unit. They see themselves as a team. They are fighters.

But they aren't fools. The Clintons know how to mount a tactical retreat. Indeed, Bill's career has been one long jujitsu match in which he's used his enemies' weight against them, falling backward to get the advantage.

So again, why is Hillary staying in?

Perhaps it's the best route to long-term victory. Washington has long swirled with rumors that the Clintons are holding some "nuclear option" in reserve against Barack Obama. The latest theory is that they've decided not to use it, as it would destroy them, too. Who knows what it might be, if it exists at all. But it's worth noting that if Hillary were the take-no-prisoners brawler everyone says she is, she would almost surely have pushed that button by now.

That she hasn't used the doomsday device buried under Clinton HQ might mean it doesn't exist. Or it might mean she's looking beyond 2008. Continued...