Here is a quote that I found telling:
[Beginning in 2009], the public perceived, fairly or not, that the Obama administration had failed to adequately address the devastating jobs situation. Consequently, the image of the Democrats as the "party of ordinary working American families" began to erode. This perception of the party as the defender of working people had been its greatest source of electoral strength for more than 70 years - and the single most frequently volunteered reason for Democratic Party identification.... [Currently, party registration is approximately equally divided, as between the Republicans and the Democrats], but how much does this new 2012 distribution of "party identification" really mean when it comes to Americans actually casting their ballots?
The answer is a lot. Indeed, probably more than any other single factor.