A century ago in the US, there were very sharp boundaries between the types of jobs that were middle class, and the types of jobs that were working class, and how much the jobs paid largely followed that, so you could know from the type of job a person had whether they were financially middle class or not. This plus classist educational and hiring systems allowed for a large degree of cultural homogeneity within a particular class. It was really easy, for the most part, to see the divisions between classes in the US. And that's largely when the cultural idea of "what middle class is" got set.
Then the labor movement resulted in a whole bunch of blue-collar jobs (i.e. jobs that involved working with your hands, trades, manufacturing, etc) getting actually paid what they're worth, which meant comfortably middle-class incomes, which blurred the lines a bit starting in the 40s and 50s.
Then, in the 70s, white collar jobs (think office work--the sort of job where you're never going to work up a sweat doing it) stopped keeping up with inflation. Up to this point, pretty much all white collar jobs were middle class jobs (at least the ones that weren't also pink-collar jobs, like secretary or teacher). So the purchasing power of the salaries for those jobs fell gradually over the decades.
But we still judge "am I middle class?" based on outdated appearances and whether or not you work a white collar job. A lot of people assume that they are middle class because they work a white collar job and live in the suburbs.
But if a single missed paycheck or major accident could spell financial ruin, you are not middle class.
To be middle class, you have to be financially stable enough that you don't have to care about things like how much groceries and gas cost. You have to be financially stable enough that you can take vacations every year without worrying about it. Not lavish vacations, necessarily, but nice ones. You have to be financially stable enough that saving for retirement is not a hardship, and neither is having a few thousand dollars in a rainy-day fund. And it's not enough to be able to afford all of that for yourself alone! You would have to be able to afford children on top of all that, either on your own income or combined income with your partner.
emergency/rainy day savings
not worrying about money issues, regular bills, the price of gas or groceries
then you are not middle class. In New York (state, not city), you have to make at least $75k/year to be middle class. In Georgia, you have to make at least $55k/year to be middle class.
And the thing is, not understanding what "middle class" actually means is really convenient for right-wing politicians. Because so many people think they are middle class when they are not. "They say this policy will help the middle class, and hurt the working class. Well, I am middle class, so I am in favor of it." Except the person who thinks this isn't middle class, and what's more, their boss may not qualify as middle class either.
