The hollowing out of the American middle class is because the huge, wealthy middle class was a post-war anomaly, from a time when the US had the only intact industrial plant in the world, and lack of communication technology and logistical sophistication meant production had to be localized and centralized. So, if you happened to be living in the right places in the US, you could have a house and a car and put a couple kids through college on an (artificially-inflated) factory worker's wage. At the same time, 80% of the population of the world was on the edge of starvation.
Now, thanks to better logistics and communications, companies can move jobs to where labor is cheaper. This has pulled billions of people out of poverty, dramatically reduced the price of goods, and generally improved global well-being--but that was at the cost of the 1% of the 1950s, which is to say the American working class. Now, if you work in a factory in the US, you only make a single-digit multiple of what a factory worker in Korea, Mexico, Germany or Italy makes (though you still have a double-digit advantage on much of the world).
It wasn't sustainable to have a tremendously wealthy middle class in a world that was mostly starving. No amount of trade barriers could maintain that: you're relying on a world market with very little competition, and the other 7 billion people aren't going to be content to sit on their hands.
What you want to do instead is to develop new, cutting-edge, high-paying industries, and thereby keep a competitive advantage on the rest of the world. Maybe you could, I dunno, develop top-notch schools to lure all the best and brightest people from around the world to your country, invite them in, encourage them to stay, and get them to innovate and create here rather than elsewhere. That might just result in whole new, massive, high-paying industries that pick up the slack left by your diminished industrial dominance.
Seems like a good idea to me! But hey, instead, you could always try slamming the door shut, chase out all the dirty foreigners, and just rely on your inherent and intrinsic American superiority to carry you forward. I'm sure that'll work just as well.