In the US middle class has always meant middle - not at top in income/wealth (mostly income) but not at the bottom.
How they obtain the money (education, job) or how they spend it (house, holiday) is not important by definition - however there is some social expectations of the American middle class.
Many Americans don't have the time off work for two holidays a year, even if they have plenty of money, which is another difference, which changes the American social expectations of the US "middle class."
A family with nurse and a teacher for example (two professionals) would bring in somewhere around $100,000/yr and be upper class.
There's a lot of variations by geography (cost of living) and stuff too, but I simplified it.
In reality the amount of financial security a family has is often a function of how well they manage it (as long as you are making above a certain amount where it becomes possible for financial security). If the above mentioned family over spent on a big house, two cars, a motorcycle, and other luxuries they consider normal, or even needed, then they can surely feel like they are living without much financial security, which doesn't "feel" upper class.
Unfortunately, middle class is a totally useless term because almost everyone describes themselves as middle class, I guess because they are somewhat out of touch with their spending and don't feel rich (previously mentioned family) or they are towards the bottom they don't want to feel poor. People who have a household income of $200,000 describe themselves as middle class. One of my coworkers, with a straight face, described me as middle class. I couldn't believe it - with a household income more than 3x the median household income in the US I can't with a straight face say I am middle class.
That's why politicians pander to "the middle class," because the majority of Americans identify with the middle class - even poor ones who don't consider themselves middle class but feel they will be there soon.
On the other side of the coin I have a friend who makes a middle class income, spends lavishly on consumer goods, and is always complaining about how poor she is.
Like I said, totally useless term.