Rasmussen has an exellent run down of the GOP Iowa primaries and what the results can mean to each top tier candidate.
Rasmussen on Iowa GOP Primaries
Among Republicans, the first big question is whether the informal network of Evangelical leaders in Iowa can turn out the base of support for Mike Huckabee. If the answer is yes, Huckabee wins. If not, the Romney will likely be celebrating. Romney has built an extensive, and traditional, political organization in Iowa but the candidate has not generated the same type of passionate support enjoyed by Huckabee.
Huckabee needs a victory in Iowa to have any chance at the nomination. Romney could survive a close second, but he will then have to follow up with a victory in New Hampshire next week. If Huckabee loses narrowly in Iowa, he will still be in a strong position to collect delegates down the road. This is particularly true in Southern States and could leave the former Arkansas Governor in an interesting position before the GOP convention.
It all depends on the religious right coming out in droves to support Huckabee. If they do, Romneys campaign, which poured millions into the state, will be given a near fatal blow. The religious right has in the past shown its ability to swing elections for its candidate of choice, and in my opinion, Huckabee will win Iowa with the help of this vital constituency. To stay in the race Romney needs to both win Iowa and defeat McCain in New Hampshire. Should Huckabee lose Iowa, he is finished.
A Huckabee victory will be a huge boost to McCain in New Hampshire. If McCain can get a respectable third in Iowa and finish second in New Hampshire, he will be in a good position to win the nomination. Fred Thompson needs to finish third in Iowa to stay in the race, a loss of third place to Mccain or even Ron Paul would be fatal. Rudy seems dead in the water, unless all the candidates tie in the early states, he has no chance.
In conclusion, the three men to watch are Romney, Huckabee, and McCain. Of these three, McCain is the most dangerous to democrats due to his high positive rating and appeal to moderates. Huckabee will bring out the religious right, which can help him win key swing states (Florida will have a marriage amendment on the ballot). His weakness is his complete marriage to the religious right, and his inability to raise cash. Romney has a big wallet but little else going for him, it is amazing tha the has to fight tooth and nail in both Iowa and New Hampshire despite outspending all other candidates put together.