By all accounts, the Iowa presidential preference on the Democratic side is pretty close between the top 3 - maybe Clinton has a slight lead. 2nd choice might make the difference. In Iowa's caucus system, voters can re-align with a different candidate if - in the first count - some don't make the cut of 15% in a precinct.
Recent polls have followed the popular belief, that the most popular choice for 2nd among Democrats in Iowa is John Edwards, followed by Obama. But, if you ask me, there's a further catch. Not everyone's second choice matters equally, only those whose first choice doesn't reach the 15% threshold. I have to believe that Clinton, Obama and Edwards will meet it in virtually every caucus site - only some tomfoolery would have their supporters switching after already viable. The caucus-goers whose 2nd choice matters most are those behind Biden and Richardson, who seem to have consequential support levels, but also seem unlikely to crack 15%. It might matter little that Edwards is the overall second choice. What will matter is if Biden and Richardson (and I suppose Dodd and Kucinich) supporters break significantly one way over the others, and what the supporters of the other 2 do to counter that. Also, will there be any last-minute deals? Remember in 2004, Kucinich and Edwards asked their supporters to switch to the other if they didn't meet 15%.
Even beyond 2nd preference, strategy can play a part. Read this MyDD post from a veteran Iowa caucus-goer to see how sometimes in a precinct you may even leave your candidate to help make another viable and decrease the number of delegates of your rival. A Clinton supporter could end up caucusing with Edwards to put him just high enough to qualify for one more delegate at the expense of Obama, for example, or vice-versa. At the end of the day, raw votes aren't important. Each precinct will be awarded so many delegates ahead of time. The job of the caucus-goers is to divide those delegates among the candidates that receive 15% or more of the support.
Most precincts award between 1 and 9 delegates. For those with 3, 6 or 9, differences in raw vote totals between Edwards, Clinton and Obama will likely be erased and they will get an equal number of delegates. But in precincts with, say, 7, small differences between the 3 will be magnified as the plurality winner - no matter how slight the margin - will likely receive 3 and the others 2. The totals we hear at the end of the night will be the count of these precinct delegates. That number may not bear resemblance to the percentage of support a candidate has, depending on how the math and the haggling shake out. I assume each of the 3 campaigns has an overall strategy - passed down through captains - of doing what it takes to minimize the delegates of one of their rivals. I believe Obama's problem in Iowa is that, I assume, he is the target of both of the others. Clinton sees him as her only real rival, while Edwards wants to become the anti-Hillary candidate. Each could send supporters to caucus with the others - or to make Richardson or Biden viable in a precinct at Obama's expense.
If you're especially interested in this, you may like playing with this caucus calculator.