A very interesting explanation of what really happens with “swing voters” when approval numbers go up and down over at the Washington Post’s Monkey Cage:
How can election polls swing so much given the increasingly polarized nature of American politics, where switching one’s support between candidates is a significant move? We investigate this question by conducting a novel panel survey of 83,283 people repeatedly polled over the last 45 days of the 2012 U.S. presidential election campaign. We find that reported swings in public opinion polls are generally not due to actual shifts in vote intention, but rather are the result of temporary periods of relatively low response rates by supporters of the reportedly slumping candidate. After correcting for this bias, we show there were nearly constant levels of support for the candidates during what appeared, based on traditional polling, to be the most volatile stretches of the campaign. Our results raise the possibility that decades of large, reported swings in public opinion — including the perennial “convention bounce” — are largely artifacts of sampling bias.
Andrew Gelman, one of the authors of the study, fleshes out the whole thing quite well.