"There are a great many on the left who ...  will not be happy if and when Mexico gets dumped..."

That won't stop it from happening, and frankly, in real terms it will mean nothing to that "great many on the left..."  IF Trudeau's team got tough and stood shoulder to shoulder with Mexico, "confronting" Trump, all kinds of s*** is likely to come down, given the man's personality quirks,  and I suspect "a great many on the left" wouldn't be at all happy with the effects of THAT on our trade.