So exactly which Democrat is ahead in Iowa? Mark Blumenthal of Pollster.com has written up a summary of the latest surveys — and the short answer is that we don't know. The biggest problem is figuring out who exactly qualifies as a "likely voter" in such an unpredictable system as a caucus.

"What makes Iowa different is that the last source of variability. It is bigger and more consequential than for other types of polls," Blumenthal writes. "So if we take into account both the closeness of the Democratic race and all sources of potential poll error, we really have no idea who is truly 'ahead' at this point in the race. The polls are simply too blunt an instrument, especially given all the uncertainty about who will participate."