I suspect the de jure, rather than (just) de facto, splintering of the country down regional lines will almost certainly result from a constitutional convention. The red states, for the most part, will likely push through an agenda that is too radical for any of the blue states to agree to. Since it is the legislatures, not plebiscites from the states themselves, that would make any decision to ratify any agreement to come from that, the more extreme agendas will be met. The reality is that there are more red states than blue states, even if there are more people in the blue states than the red ones.
There are three scenarios I see from that:
1) Four or possibly five separate countries emerge from this (Texarkana (Texas and the Plain States), Cascadia (PacWest), Dixie, Inland Empire and New England) after regional coalitions form.
2) Canada merges with the Blue States (and probably Alaska).
3) The Pacific regions of both the US and Canada merge, Alberta (and possibly Manitoba and Saskatchewan) become part of Texarkana, Quebec gains its independence, and the contiguous EastCoast and Upper Midwest merge with Eastern Canada (Ottawa and the Maritimes).
The one scenario I don't see coming out of such a convention is that the US will remain whole.