The Cellphone-Only Vote

Silver tries to make sense of conflicting polls. One possible explanation:

Roughly one third of American households rely solely on mobile phones and do not have landlines, meaning they will simply be excluded by polls that call landlines only. Potential voters who rely on cellphones belong to more Democratic-leaning demographic groups than those which don’t, and there is reasonably strong empirical evidence that the failure to include them in polls can bias the results against Democrats, even after demographic weightings are applied.