Pew has a helpful study up about the evolution of the middle class over the past ten years, documenting some of the ways in which it has lost ground. Perhaps most striking is the following finding: in 1971, middle income households had 61% of the national income, but, by 2010, these households had only 45% of the national income. Upper income households went from having 29% of the national income in 1971 to 46% of the national income in 2010. Interestingly, most of this change in income share seems to have taken place between 1980 and 2000. 2000 also serves as the tipping point after which economic growth slowed down for most sectors of Americans.
The connection between declining fortunes of the middle and the economic slowdown is a point that could be further explored by conservatives.