That was one policy wonk yacking in front of the Senate.
One of many, here's some of what Brookings Institute says:
http://www.freerepublic.com/focus/f-news/1379606/posts
All three countries also will have to confront issues of sovereignty and basic nationalism in deciding how much further integration to pursue. Virtually by definition, multilateral cooperation leads to the loss of sovereign decision-making power. In exchange for expected benefits, NAFTA has required the US, Mexico, and Canada all to substitute mutually-agreed upon rules and joint dispute resolution procedures for what had previously been national decisions. Deeper North American integration will introduce more common rules and regulations, more joint decision-making, and perhaps even new trilateral institutions. These changes will provoke opposition among groups in all three countries who will use traditional conceptions of "sovereignty" as their defense.