Free trade over protectionism is like the one thing economists agree on.
Economists have the least influence on policy where they know the most and are most agreed; they have the most influence on policy where they know the least and disagree most vehemently.
If there were an Economist's Creed, it would surely contain the affirmations 'I understand the Principle of Comparative Advantage' and 'I advocate Free Trade'
Which economists? Libertarians economists? Liberal economists? Socialist economists? Neocon economists? Economics is a pseudoscience, every economist only agrees with economists of his own ideology and since it's obvious that the higher US private educational system promotes neocons and liberals that why most of them agree on it.
In terms of which ones agree on the principles of comparative advantage? All of them really.
I am sympathetic to an extent to your underlying criticism though. Free trade is not really free trade as we often practice it. China still operates a very mercantilist economy in many sectors. Then again so do we. However, what Trump has hinted at is even worse. Assuming he doesn't pivot. At least if I am understanding his tax right. My hot take is it will hinder American competitiveness with no real advantage without raising import tariffs, which if done would trigger trade wars.
US manufacturing is up. Re-shoring has been an ongoing trend and businesses have been in a bit of re-thinking on the profitability of international expansion. But there are just some hard truths people don't want to face like automation. Which accelerated during the recession. And no matter how punitive your tariffs or taxes are, it is not going to stop that.
You want to make major gains to grow the middle class? Fix fucking healthcare properly and invest in the already high demand healthcare sector where 13 of the 20 fastest growing occupations are. Where the outrageous rise in insurance costs have factored in stagnating wages. Invest in re-education and trade schools. Invest in infrastructure directly, build out the green energy sector.