bunch of graphs and tables that come from The Gamble, a book on the 2012 election by John Sides and Lynn Vavreck. The thing that struck me about all of this data was how set in concrete everything seems to be, with very little possibility of change. Consider the table above, which shows no change to speak of despite nearly a year of campaigning between December 2011 and November 2012. Those who were undecided in 2011 split almost evenly between the two candidates; it looks like Obama got a few more defectors from Romney than vice versa, but that may be a statistical quirk. Basically people vote based on their own self-identification (as liberals or conservatives) and on their general sense of how the country is going. The campaign made no difference.
But that is nibbling around the edges. The minds of 90% of Americans are made up before the campaign even begins, and they are set in their ways. Another graph Klein presents shows that people hardly changed at all between 2008 and 2012, except that Obama lost 1% of every group. And this explains why Democrats are so excited about demographic change -- since the voters show no sign of ever changing their minds, the only way they can get more votes is if the country becomes more Hispanic and more Asian.