– Andrew Gelman nailed it:

In summary, &#8220-momentum&#8221- can exist, but the places where you&#8217-ll see it is in races where current public opinion is out of step with best predictions. The mere information that a race has a 5-point swing is not enough to predict a future shift in that direction. As Nate emphasizes, such a prediction is only appropriate in the context of real-world information, hypotheses of &#8220-factors above and beyond the direction in which the polls have moved in the past.&#8221-