Tuesday's Iowa caucuses are two contests for the price of one.

Mitt Romney, as he has all year, faces off against himself. He's in the enviable position of being able to win even if he loses - unless he loses spectacularly.

The rest of the field will jockey with each other for the only other position that matters: the non-Romney candidate. One of them may win that mantle, yet all of them might lose.

The judgment of some 120,000 Iowa Republicans is unlikely to lead directly to any candidates dropping out. The compressed early calendar means other contests loom for all of them, and the prospect of two debates in New Hampshire over the weekend can give even the most beleaguered candidacy hope.

But functionally, candidates who had once banked their hopes on strong Iowa showings cannot afford to slip too far down the ladder.

Finishes of fifth or worse for Rick Perry or Michele Bachmann could effectively doom their candidacies, even if they continue to march on to South Carolina and beyond, as both are vowing to do.

The grouping of the top tier of candidates will contain much of the night's intrigue. Romney is virtually guaranteed a spot near the top of the leaderboard - he's atop the latest Iowa polls - and can afford some slippage, given his history of harsh judgment by Iowa caucus-goers, and his commanding lead in next-up New Hampshire.

A Ron Paul victory, or close second-place finish, won't make him any more likely to win the nomination. But it could embolden Paul to stay in the race longer, or even pursue a third-party run as a libertarian - a development with dramatic implications for the GOP nominee.

Rick Santorum stands the best chance at an upset victory. He's showing signs of consolidating social conservative support - a force that could be a potent weapon against Romney as the campaign progresses, though Santorum needs to put together a campaign infrastructure with lightning speed to be able to take advantage of it.

That leaves Newt Gingrich, whose numbers have gyrated the most in recent weeks. He seems resigned to the fact that the negative advertisements run by Romney's allies against him have taken a significant toll.

But even as he concedes likely defeat in Iowa and New Hampshire, Gingrich is vowing to take the fight more aggressively to Romney in the days and weeks that follow. It's an acknowledgement that his non-aggression was a mistake, but also a calculation that it's not too late to do damage to a still-vulnerable frontrunner.

For all his flaws - and even taking into account his plummet in the polls - Gingrich is the candidate who looks best able to compete with Romney over a prolonged stretch of the campaign. A strong finish in Iowa, though, is critical to setting that in motion.

In any event, the Iowa caucuses will clarify the direction of the Republican Party, particularly the willingness of rank-and-file members of the GOP to compromise on their ideals in the name of unity and electability. And the voting will signal how quickly a party that's desperate to beat an incumbent president is likely to be able to settle on its choice.