Beyond that, it is worth noting that no pre-primary polling mimics the actual nomination process. The Iowa caucuses are unique in that Republican caucus goers might go in planning to support one candidate, but come out supporting another. The reason is that, in the caucus meeting, participants listen to campaign pitches for each of the candidates. So, weakly felt preferences – the very sort that the Des Moines Register poll is finding today – could very easily change. After Iowa, primary results often depend very heavily on momentum, or the phenomenon wherein a candidate’s victory in a previous contest increases his chance of victory in the next contest. Momentum is closely related to the fact that primary voters know very little about the specifics of each candidate, which makes winners seem especially appealing. Again, this is a dynamic that pre-primary polls simply cannot capture.
So, I would recommend extreme caution in interpreting polls such as the Des Moines Register poll, so early in any presidential cycle. But for this cycle, more prudence than normal is warranted – this is the most open Republican nomination battle in the modern era. If ever was there a year that a candidate was to surge from nowhere to capture the nomination – like Jimmy Carter in 1976, for instance – it looks to me like it will be this year. That would make early polling even more dicey.