The middle class are the ones who fluctuate between the two categories: They have to keep working, but as long as they do, they have some surplus income. If they spend too much and go into debt, their surplus income goes away, and they have to work more.
This is useful because it explains how some can consider themselves to be "working class" or "middle class" and yet have income that is both above average and above median. They still have working class (or middle class) anxieties, mostly because of their local economy or their bad spending habits.
The important thing I try to always remember is that it's the 'middle' class, not the center class. Civilization can't survive with mostly middle management and mechanics - somebody has to actually do something at some point, rather than designing something, directing something, or fixing something. The middle class is a buffer between the bulk of people who work with their bodies, and the tiny number of people who try to maximize the ratio of those people's output vs. their compensation.
The middle class has never characterized America, it's just the class that writes all of the characterizations.
My experience is that most college-educated people don't really understand this, because they look at themselves and everyone they know and extrapolate their definition of "the middle class" from what that population makes, which skews their perspective considerably. So you get discussions about how people making $100,000/year or $250,000/year are "middle class," when statistically speaking they are very much not.