From the excellent Scott Winship:
Krueger’s claim of a shrinking middle class relies on the same peculiar definition. Specifically, “middle class” is defined as having a household income at least half of median income but no more than 1.5 times the median. I re-ran the numbers using the same definition and data source as Krueger and found that the entire reason the middle class has “shrunk” is that more households today have incomes that put them above middle class. That’s right, the share of households with income that puts them in the middle class or higher was 76 percent in 1970 and 75 percent in 2010—two figures that are statistically indistinguishable. For that matter, I am not discovering fire here; Third Way made the same point in early 2007 (page 7). A shrinking middle class is only a problem if it reflects fewer people reaching the middle class.
The post is excellent throughout and it contains many more points of interest.