Some in both the US and Canada want to negotiate a new US-Canada free trade deal apart from Mexico. This would entail several awkward factors:

1. In its current frame of mind, the US would make would make many demands that Canadians would find unpalatable and would reject many that Canadians consider important, including several currently imbedded under NAFTA. 

2. There would be strong US lobbies to simply cancel NAFTA unless all US demands were met.  In response, Canada would have to be prepared to bargain hard and long (including being prepared to reject a new agreement if the terms were too unfavorable).

3. A protracted period of anxiety and uncertainty would exist pending the final outcome of such negotiations.

4. In short, Canada currently is deeply intertwined economically with both the US and Mexico and we would be at a disadvantage negotiating a new treaty  at this time from scratch.