For me the answer about the "American middle class" starts with the recognition that versions of this question are being asked in every single country around the world. Obviously the rich-country version of the question is different from what people are asking in becoming-richer economies in Southeast Asia and Latin America, and in still mainly poor economies in parts of Africa and the Middle East.
Bernie Sanders's problem with exports, "because they benefit large corporations," is more than somewhat astounding. Who does he thinks works in all the Boeing plants? Doesn't he realize that a large percent of Boeing shares are owned by various American retirement funds and individual American 401k plans?