Rather than measure middle-class by income, the proper examination procedure is consumption.
Consumption means what can be spent on a daily basis achieving a defined life style, lack of debt, child education, retirement, and inter-generational wealth.
Earning power for middle-class means defining what kinds of jobs are worthy of society allowing middle-class consumption.
How does technology affect the size of the middle classa?
How does political influence create middle-class entry and existence? Who pays for these politically influenced consumption levels? Does this create a separate class of people the 14th Amendment forbids?
Expanding the "consumption capable" middle class through taxation of the wealth accumulaters to insure capitalism. Why is the "consumption capable" middle class different from the "income" middle class?