the reason chretien signed nafta in the first place - despite campaigning against it - was that he realized that a bad deal was better than no deal, and he was right, at the time - because the deal included mexico, and we couldn't be left out of it (they'd have taken everything from us).
now, we're talking about making an already very bad deal an even worse deal. and, it still might be right that making a very bad deal an even worse deal is still better than there being no deal, if that bad deal still includes mexico.
but, there comes a point where that doesn't hold any more and you have to say "enough".
i'd be leaning towards slippery slop arguments on this, and arguing that we can't let it continue to slide, negotiation over negotiation.
what we want is a bilateral deal, whether the prime minister understands it or not.