"I can't for the life of me figure out why someone like [Rick] Perry [is] staying in the race," said a strategist for a rival campaign. "I think part of it is how the race played out from the straw poll to the [Iowa] caucus. Everyone got a chance to be in the lead.... Thus, those who are staying in believe they have a chance to catch lightning. I don't see it, but hope dies last."

Indeed, every gyration of the race thus far has rewarded sticking it out when things seemed hopeless: Santorum was polling in single digits a week before the caucuses, only to surge at the finish and tie Romney with 25 percent of the vote. The proliferation of debates has increased the opportunities for sudden turning points, and with them, the lottery-like sense that at any moment, fortune's favor might turn in a new direction. One minute you're Herman Cain, long-shot candidate on an extended book tour; the next, you're Herman Cain, first place in the polls; the next, you're Herman Cain, disgraced ex-candidate, and it's someone else's turn in the revolving door.

It is always possible that something terrible could suddenly happen to Romney. But it is usually a bad sign for your candidacy if you're only sticking around as an understudy.

Perry appeared to have gotten the message after his fifth-place finish in Iowa, saying in his concession that night that he would go back to Texas and reassess his campaign. But he woke up the next morning determined to continue, surprising his own staff by announcing on Twitter he was headed to South Carolina.

Meanwhile, in New Hampshire, Perry received less than 1 percent of the vote. Though his debate performances have improved from their dark nadir, his attacks on Romney's business credentials as "vulture capitalism" have not appeared to resonate with South Carolina Republicans.

"Perry obviously has not made the sale, even with a week's jump on everyone else," said Ed Rollins, the veteran Republican strategist and onetime Michele Bachmann campaign manager. "They all want to pretend the polls don't count, but polling is pretty darned accurate -- look at the polls so far. You can't move those kind of numbers in a week."

Perry advisers privately acknowledge that, barring a plane crash that eliminates the rest of the candidate field, they are more or less playing out the string. Ever since the Texas governor compared his South Carolina stand to the battle of the Alamo last week, it has seemed apt.

"This is his Alamo," said Chip Felkel, an unaligned South Carolina strategist who worked on George W. Bush's campaigns in the state. "To save face so he can go back and govern Texas, he's got to at least stay in through South Carolina." Perry has been endorsed by some notable figures in the state, including the speaker of the House, who may feel he owes it to them to give it his all, Felkel speculated.