It would very slowly escalate toward nuclear, and then all at once.

Nuclear rhetoric would escalate rapidly by the losing / weaker side.

It's a common fallacy that conflict between two nuclear powers would instantly jump to nuclear exchange. That isn't how it would go at all, although the propaganda that it would does likely help to prevent wars. Nobody is interested in vaporizing all of their own people by starting a nuclear exchange from the first bullets fired. There would be various high-intensity inflection points, triggers, that would risk nuclear escalation: when one side or the other starts losing a lot of territory; when one side or the other loses a major city; when one side or the other is at risk of operational collapse (leading to more rapid losses in the field); when one side or the other is at risk of losing their core territory / capital; and so on. The key inflection points would be prime candidates for nuclear threats, to try to get the winning side to stop or back off immediately or else. From there it'd be a complex equation as to when a side would actually finally set off a nuke (would it be a warning nuke first, etc).