The socio-economic stratum below the middle class tier.
An oddly formed term, given that the most commonly conceived social hierarchy has three tiers (upper, middle and lower) and that proper English indicates the use of the superlative in cases where more than two items are within the relevant group. "Lowest class" seems the more appropriate formulation. However, if one takes the middle class as the frame of reference, the the comparative can be used within the rules of English grammar.
Another note on usage: "lower class" (also "lower-class") should not be confused with "low-class," a colloquial adjective used in the vernacular to describe anything of ill repute, unseemly origins, mean taste, vulgar, et cetera.