A former economics teacher "Wonks Anonymous" does a mea culpa about trade theory:
I believe that there is a fundamental problem with the analysis of free trade that most economists - myself included during my teaching career - present.
When a country has a relative scarcity of labor - like the US today - there are two impacts from trade with a labor abundant world: We see a slight gain in national income and a large redistribution of income from labor to owners of capital.
When faced with this result of trade theory we, as economists, have regularly waved our hands and mumbled something about the "winners" paying off the "losers" out of the overall gains.
Except that we never seem to see the winners paying off the losers.
The problem is that the gain in income is small - it's been years but my best recollection is that the estimated gains from NAFTA did not exceed 1% of GDP. It is a classic example of a "Harberger triangle". At the same time the redistribution is relatively large.
The political motivation for free trade is all about the redistribution and, as economists, we simply enable this redistributionist movement with our arguments about "efficiency"