In 2004, Dennis Kucinich told his supporters to caucus for John Edwards if Kucinich was not viable in their precincts. (Here's an explanation of viability and the caucus system.) That recommendation helped propel Edwards to a second place finish in Iowa.
This year, Edwards won't receive the same boost. Barack Obama has secured Kucinich's second-place recommendation. Not every second- and third-tier candidate will identify a leading candidate for their supporters to caucus for should he or she fail to meet the 15 percent threshold for viability. But for some, it's not hard to see where there supporters will go.
According to Pollster.com, Obama, Edwards, and Clinton are the only candidates polling over 15 percent. Here are the numbers:
Clinton — 29.4 percent
Obama — 27.0 percent
Edwards — 25.1 percent
Richardson has 5.5 percent; he has not indicated who he wants his supporters to caucus for should he be unviable, which makes sense because Richardson has the strongest chance of being viable of the second-tier candidates. If a Richardson supporter were trying to make up his or her own mind, he or she might see Richardson's strong anti-war stance and look to Edwards or Obama. But he or she might look at Richardson's lengthy resume and decide to support Clinton. We'll call that a wash.
Biden is at 4.4 percent. His supporters like his experience and foreign policy credentials. If forced to choose one of the top three to support, they will likely support Clinton. (Though perhaps Edwards.)
Dodd and Kucinich poll at 2 percent or below in most polls. Gravel polls at zero. Their supporters will likely be spread across the top three, with a heavier proportion going to Obama and Edwards. The Kucinich-Obama agreement is obviously in play there.
There are a couple other factors to keep in mind. Edwards has been in Iowa forever, and he did well there in 2004. Iowans seem very comfortable with him. From some second-place polling I've seen online, he will likely benefit the most from second-choicers. Many voters are ABCs, anybody-but-Clintons. It is widely assumed Clinton will gain the least from second-choicers.
What does this mean? Clinton, Edwards, and Obama may all get just about 30 percent. Don't trust anyone who tells you they know who's going to win this thing.