By establishing the principle that U.S. corporations could relocate production elsewhere and sell back into the United States, NAFTA undercut the bargaining power of American workers, which had driven the expansion of the middle class since the end of World War II. The result has been 20 years of stagnant wages and the upward redistribution of income, wealth and political power.
Full article:
http://www.epi.org/blog/naftas-impact-workers/
Interestingly, Trump and Clinton share the same views about NAFTA. Former President Bill Clinton admitted he thinks it was a mistake.