Until now the American interests that have been rattling the Republican cage are those that are negatively affected by NAFTA (such as the softwood industry).  Obviously powerful American interests that benefit from NAFTA had little need to raise their voices until now.  I suspect Trump may have an enormous battle on the home front as those powerful and rich voices start speaking up about the benefits of free trade for their industries and their American workers.  Whether we keep the agreement as is, renegotiate it, or throw it out there are interests that win and lose on the American side and the Canadian side.