One last observation from the preternaturally savvy Nate Silver on what is a hobbyhorse of my own:

The cellphone effect. This one is pretty simple, really: a lot of American adults (now about one-quarter of them) have ditched landlines and rely exclusively on mobile phones, and a lot of pollsters don’t call mobile phones. Cellphone-only voters tend to be younger, more urban, and less white — all Democratic demographics — and a study by Pew Research suggests that the failure to include them might bias the polls by about 4 points against Democrats, even after demographic weighting is applied.

There is also some indirect evidence for the cellphone effect. What follows is a list of each firm’s final generic ballot poll, arranged from the best result for Democrats to the worst:

You can see that there is a rather strong relationship between whether a company included cellphones in its sample or not and the sort of result they showed. The polls that were conducted without cellphones showed Republicans ahead by an average of 9.3 points; those with them showed a smaller, 4.8-point advantage. That’s a difference of 4 or 5 points (and one that is statistically significant at the 95 percent confidence threshold), which is about of the same magnitude that Pew identified.