Trump can't cancel NAFTA...
Via an executive order (which we all know he has a huge hard on for) he sure can force the hand of Congress - would they stand up to him and block it, or would they just rubber stamp his wishes for fear he turns on them once again?
I'd like to think that Congress understands the potential market nightmare it would create and block it, but this government doesn't exactly act sensibly 100% of the time, do they?