In the USA, this is compounded by the belief by the majority of the population that they're in the upper middle class (or will be Real Soon Now).
I'm pretty tired of hearing this temporarily embarrased millionares trope. The middle-class, poor, or what have you that vote in droves for policies that clearly benefit the wealthy do not do it because they believe they will be wealthy themselves one day; they do it because the TV, Internet, and print media they choose to view tells them it's the fair or moral thing to do. We Americans do like to feel that we are the good guys, even if we may be misguided or misled.
And while the middle class may be disappearing on paper, people who "believe" themselves to be middle class, or upper middle class, likely do so because they are living a comfortable lifestyle and as such have no reason to think that they are poor. Just because the wealthy have greater wealth now than they did in the past (shifting the mean income) doesn't mean two cars, a tv, and the means to feed your children is no longer middle class.