"Earlier, we published an article explaining why there is no such thing as a scientific poll. In a nutshell, because polling relies on good but sometimes inaccurate assumptions, it is far more art than science. As we noted, "Tweaking [voter] turnout models is more akin to refining a cake recipe than doing a science experiment." Still, since American pollsters are good at their jobs, polls tend to be correct more often than not."
Nate Silver's 'Chance of Winning' Election Forecasts Are Complete Nonsense