If we hadn't had so much low skill immigration especially illegal immigration, we would have had serious reduction in the size of the "lower" class.
Half of illegal aliens have no high school degree versus 10% of natives. Their American born children graduate at higher rates but not a lot better. So you get three generations before they start to assimilate and hit middle class, in many cases. That means that enabling illegal immigration has contributed to the high percentage in poverty - compounded by immigrants depressing wages for the bottom 25% of native born Americans.