What's a "socioeconomic class", in this sense? I understand what the terms denotes in a very general sense, obviously, but you seem to be describing that as positive, at least very broadly coherent entities, rather than simply as sociological categories, which complicates it a bit.
I don't think that categories have hard and firmly defined lines, if that's what you mean. Unlike a nation that has an aristocracy that actually rules country and has real privilege and power, places like the US have both blurrier borders and more fluid ones. Now leaving the racial issues for another discussion, we have the dirt poor, those that are homeless, the welfare dependent, the working poor, an extremely broad and inclusive "middle class", the sorta rich, the rich, and the extremely rich.
Class warfare in the US in recent decades has been the top 3 groups at the expense of the rest. Class warfare in political rhetoric has been the top groups accusing anyone who wants to help the rest of warfare.