538 on primary polls:
The other big difference between the general election and primaries is that polls are not very reliable in the primaries. They improve as you get closer to the election, although only up to a point. But they have little meaning now, five months before the first states vote.
It’s not only that the polls have a poor predictive track record — at this point in the past four competitive races, the leaders in national polls were Joe Lieberman, Rudy Giuliani, Hillary Clinton and Rick Perry, none of whom won the nomination — but also that they don’t have a lot of intrinsic meaning. At this point, the polls you see reported on are surveying broad groups of Republican- or Democratic-leaning adults who are relatively unlikely to actually vote in the primaries and caucuses and who haven’t been paying all that much attention to the campaigns. The ones who eventually do vote will have been subjected to hundreds of millions of dollars’ worth of advertising, had their door knocked on several times, and seen a half-dozen more debates. The ballots they see may not resemble the one the pollsters are testing since it’s likely that (at least on the GOP side) several of the candidates will have dropped out by the time their state votes.