Iowa Caucus
01/01 PPP (D) -- Paul 20, Romney 19, Santorum 18, Gingrich 14
01/01 Insider Adv -- Romney 23, Paul 22, Santorum 18, Gingrich 16
12/30 Des Moines Reg -- Romney 24, Paul 22, Santorum 15, Gingrich 12
12/29 WeAskAmerica -- Romney 24, Santorum 17, Paul 14, Gingrich 13
12/28 NBC/Marist -- Romney 23, Paul 21, Santorum 15, Perry 14
12/28 ARG -- Romney 22, Gingrich 17, Paul 17, Santorum 11
12/28 Rasmussen -- Romney 23, Paul 22, Santorum 16, Gingrich 13
12/28 Insider Adv -- Romney 17, Paul 17, Gingrich 17, Santorum 13
12/27 CNN/Time -- Romney 25, Paul 22, Santorum 16, Gingrich 14
12/27 PPP (D) -- Paul 24, Romney 20, Gingrich 13, Bachmann 11
It's impossible to say for certain who the winner will be. Romney has a very slight lead in most of the polls, has organization, and has the best electability argument. Ron Paul is close behind, and his supporters are intense. Rick Santorum is in third, but has the late momentum and is the most likely to benefit from the coalescing of low-polling Rick Perry and Michele Bachmann. Despite trailing by a few points, Santorum may actually have the edge.
Taking first in Iowa would be a boost for any of the three leading candidates. The size of the boost usually depends on whether the candidate exceeded expectations, and the size of the margin of victory. In this case, no one knows which candidate will win. The only expectation is that it will be close.
In one sense, Santorum is likely to benefit even if he doesn't take first place: He's ahead of other Anti-Romneys like Gingrich, Perry, and Bachmann. He'll have an argument to stay in the race. Ron Paul is going to stay in the race no matter what, and a victory for him would only serve to de-legitimize the Iowa Caucus.
Romney's team will have the most flexible spin on the results. Romney carefully avoided expending too many resources in Iowa, because he wanted to keep expectations low. But he also spent enough to make it possible to win. Obviously a win in Iowa would be an enormous boost to the idea that Romney is the frontrunner. A second-place finish, especially if it's close, would also be taken as a win. If Santorum wins, Romney's campaign would argue something to the effect of, "Iowa is an Evangelical state, and the Evangelical candidate only barely beat Romney." If Paul wins, Romney's campaign would argue that Romney was the highest-ranking sane candidate, and that Iowans should be disregarded. A third place finish behind both Santorum and Paul would be disappointing, but if Paul is close to winning, that fact would be used to attack Iowa's credibility.
Among the lower-tier, it will also be competitive: Bachmann wants to beat Perry to claim her status as the chief Tea Partier. Perry wants to avoid that, and would like to beat Gingrich to claim his status as the chief Southerner to compete in South Carolina.
