As you read this, Republican voters are gathering in precincts across Iowa to choose the winner of the 2012 presidential campaign's first caucuses.

Eight candidates are competing, and the latest Des Moines Register poll showed them finishing in this order, with a statistical tie for first place: Mitt Romney, Ron Paul, Rick Santorum, Newt Gingrich, Rick Perry, Michele Bachmann, Jon Huntsman and Buddy Roemer.

But when it comes to Iowa, which has a spotty track record of predicting the eventual nominee, who loses is almost as important as who wins. It has been said that the main function of the caucuses is to weed out the long-shot candidates before the real race gets under way.

Recent elections bear this out to some extent: Democrats Joe Biden and Chris Dodd dropped out after the Iowa caucuses in 2008, Democrat Richard Gephardt in 2004 and Republican Orrin Hatch in 2000. But there are notable exceptions in which the Iowa caucuses did not narrow the field at all. In 2008, for example, the Republican candidates who performed poorly in Iowa stayed in the race through at least five more contests. Fred Thompson, who finished third, and Duncan Hunter, who finished seventh, withdrew after South Carolina. Rudy Giuliani competed through Florida, and everyone else competed at least through Super Tuesday.

What will the day after look like this year? Will anyone drop out after Iowa, and if so, who?

The Leaders: Romney and Paul

Romney will not drop out even if he places significantly lower than expected, because he has a double-digit lead in New Hampshire, strong infrastructure elsewhere and, most importantly, a campaign chest large enough to cover the costs of competing in all 50 states. For him to withdraw would be unthinkable.

Paul will not drop out either, even if he finishes lower than expected, because he has a passionate and growing base that will support and fund him for the long haul. Although he remains in the middle of the pack nationally, his grassroots efforts are unrivaled, and those efforts will carry him well into the primary season, even if he does not end up winning the nomination.

The Dark Horse: Santorum

A week ago, Santorum seemed the most likely bet to drop out after Iowa -- but that was before he pulled off a remarkable last-minute surge from sixth place into third.

The Iowa caucuses are, as the saying goes, an expectations game. Candidates don't necessarily have to win the caucuses to win the post-caucus calculus: Exceeding expectations can be just as important.

Given that this time last week, even Santorum's supporters doubted he would finish higher than fifth or sixth place, a third-place finish -- especially if it put Santorum ahead of Gingrich, who was the front-runner until recently -- would be a major upset. This would allow Santorum to justify continuing his campaign in spite of what he has acknowledged is a shoestring budget, and in spite of his limited appeal beyond Iowa, where the Republican electorate is more socially conservative than average. It would not, however, justify continuing his campaign for the long haul.

If Santorum performs as expected tonight, he will probably stay in the race at least until the South Carolina primary, but after that, his campaign will not be able to support weekly, multi-state primary campaigns that fill the calendar through June, Jamie Chandler, a political scientist at Hunter College in New York, told the International Business Times.

Santorum's goal is to disprove the argument that his very conservative platform cannot catch on beyond Iowa. We feel very, very good that we've got the organization, and money is coming in better than it's ever come in, he told ABC News on Tuesday. And when we do well tonight, we suspect we'll have the resources to be able not just to compete in New Hampshire, but to compete all the way through.

All the way through to the Republican National Convention is a dubious goal -- it would take an upset of historic proportions for Santorum to win the nomination -- but unless he crashes as suddenly and spectacularly as he rose, he will almost certainly stick around for a little while after Iowa.

The Fallen Front-Runner: Gingrich

If Iowa is an expectations game, Gingrich is the clear loser. Three weeks ago, he led Romney and Paul by double digits, peaking at 31 percent support. Now, polls show him in fourth or even fifth place, and he will be hard-pressed to make it back into the top three.

But if Gingrich has proven anything about himself, it is that he is singularly stubborn. He pushed doggedly on after his campaign staff quit en masse last spring, and he continued pushing through several months of single-digit poll numbers before becoming the candidate du jour in November. After all that, it is hard to see him dropping out after one poor showing.

Gingrich has fallen in New Hampshire, too -- Paul has surpassed him for second place, and he is perilously close to ceding third place to Huntsman -- but he still holds a slim lead nationally and larger leads in South Carolina and Florida. Although the most recent polls in those states are from two weeks ago, they are enough to justify him staying in the race through a few more contests to see if his fortunes improve.

Gingrich's campaign is still in play if he performs well in the South Carolina and Florida primaries, Chandler said. South Carolina is the make-or-break-it race for Gingrich. If he doesn't win there on Jan. 21, he'll be out of the race.

The New Hampshirites: Huntsman and Roemer

Below Gingrich are the stragglers: Perry (11.5 percent in the RealClearPolitics average), Bachmann (6.8 percent), Huntsman (2.3 percent) and Roemer (not included in most polls).

Huntsman and Roemer will not drop out after Iowa no matter how low they finish. Huntsman never planned to do well in Iowa anyway -- he has thrown almost all of his resources into New Hampshire, where he has a chance to finish third -- and Roemer, a former governor of Louisiana, has said that his goal is to get 5 percent of the vote in New Hampshire, which would qualify him for the next GOP debate. If they do poorly there, though, that is when the real winnowing speculation will begin.

That leaves us with two candidates who might drop out after Iowa.

The Potential Drop-Outs: Bachmann and Perry

Bachmann and Perry will likely be out of the race after tonight, Chandler said.

Both candidates have said explicitly that they plan to compete through South Carolina no matter what happens in Iowa.

We bought our tickets to go to South Carolina, so we're going on. We'll be on the plane to head to South Carolina, Bachmann told CBS News on Monday. She added at a campaign stop on Tuesday, We're moving forward because this election is far from over. This is the opening chapter. Tonight is the first vote. We've got a long road to go.

Perry struck a similar note in last-minute campaign stops in Iowa, calling himself the only candidate who is both a proven conservative and has the money and organization to wage an extended, national-level campaign.

His campaign, he claimed, has the ability to raise the money to go all the way through.

Will Anyone Drop Out?

Of course, no candidate will say on the eve of the Iowa caucuses that he or she might drop out afterward. Everyone has to project an air of confidence and persistence until the votes are counted, and insisting that they won't drop out doesn't mean they actually won't. But once the results are in, the candidates who performed poorly will have to grapple with both political and financial realities.

These early victories have a tendency to build momentum not only in terms of support but in terms of money, Costas Panagopoulos, a political scientist at Fordham University in New York, told the International Business Times. I think that if we see someone like Bachmann, for example, or even Rick Santorum -- if they don't do very well, they could consider dropping out if the writing is on the wall and they don't have the resources to pursue subsequent victories effectively.

After New Hampshire and South Carolina, we will almost certainly see several candidates drop out.

But that may not happen after Iowa. In this race of all races -- a race in which every single candidate except Huntsman and Roemer has polled in the top three in Iowa at some point, and in which no front-runner has lasted for much more than a month -- it is not hard to believe that even the longest of long-shot candidates will stick around for a few more rounds.