The middle-class of America is not monolithic. By the accepted definition, the middle-class encompasses the largest segment of the American public. It is a vague demographic of financial solvency that is without doubt the most diverse of the three vaguely defined economic classes.
Name the demographic; base your assessment of the middle-class on age, race, religion, profession, level of education, number of children, property ownership, political and social activism, level of credit card debt or the preference in household pets and you'll realize that the term 'middle-class' encompasses such a wide range of disparate groups of citizens that the term is nearly meaningless. One might as well simply say 'folks' or 'people'.
'Middle-class' lacks specificity. It is a vapid, empty term used by pundits and politicians as an expedient for prattling on while saying nothing substantive.
Speaking of which...
When are any of these candidates actually going to say anything of real substance about the issues?
Perhaps when the mean temperature of the mythical nether-regions falls below zero degrees Celsius for a protracted period.