Obama and Hilary in their elections campaigns in Ohio and Texas expressed their intentions to renegotiate NAFTA. It might have worked in their favor especially in Ohio where they are pandering to the rust belt who blame nafta for taking away their jobs. Canada and Mexico have expressed their utter discontent in the democratic candidates intentions. Canada supplies more 1.8 Mbbl/d of oil to the US which accounts for more that 10% of the US consumption. Canada has retaliated by saying that they would pull out of the agreement if it were to be renegotiated and this would not restrict them from pursuing other avenues of revenue for their vast oil reserves. China has shown great interest in Canadian oil with the recent developments.
Also $380 billion are gained anually to the manufacturers and farmers that are directly affected by the trade agreement. Mexico would also be greatly affected if NAFTA were renegotiated and there would potentially be an even bigger splurge of illegal immigration.
I believe that the view of both Democratic candidates with respect to changing the clauses of the NAFTA agreement is extremelt short cited and needs to be re-thought.