Answer:
So this is a good question to start.
The main reason that U.S. Labor unions were opposed to the participation of NAFTA was because it meant US made productions would actually be out-sold by other countries.
Typically, goods that come from out of the country end up costing the buyer less, so they will end up buying from other trading companies, sense it is often cheaper.
Hope that helps and I hope you have a great day!!!