Read the Constitution please! the president can negotiate on foreign deals but he can not put them into law...
Do you really think that is the issue? NAFTA was approved by your legislative bodies during the Reagan years. It carries both the weight of your legislative bodies and the reputation of your government to honour deals it has signed. And I know there are far more people of honour on this site, who understand honour and integrity, than on the Dummie site.
If you expect other countries to honour deals they have signed with you, you must honour the deals that you have signed or you formally abrogate them. Ignoring them because some special interest groups are aggrieved causes the type of trade disputes that are occurring now.