The people who are most harmed by NAFTA are those in the various countries whose sectors of the economy are opened up to competition that is too much for them.
For example, unskilled workers in the United States have a hard time competing with workers in Mexico. The reason for this is that workers in Mexico will work for much lower pay and the workers in the US do not have any special skills that the Mexican workers lack. As another example, farmers in Mexico have been harmed by competition from farms in the US. Farms in the US are highly mechanized and industrialized and are therefore much more efficient. Mexican farmers cannot compete with these farms on price.
Since NAFTA opens economies up to more competition, it exposes some sectors of economies to competition that they cannot defeat. It is the people in these sectors who are the main losers from this treaty.