A lasting national divorce won't happen. Even if you could divy up some territory, eventually the left would infiltrate the institutions of the right and would be right back to where you started. The problem is psychological. The pathology of leftism includes narcissism and a heavy dose of envy. It's a toxic combination where they cannot accept that there are neighboring people that believe they are ideological failures (which they assuredly demonstrate every time they achieve some level of authority) while simultaneously cannot abide the success and wealth of those same people. They will be driven to destroy such a neighbor just as they do right now.
And before you can say they won't get past the guns on the walls, realize that leftism can sprout within the walls like a fungus and grow from there. The violence comes later.
About the only thing you can do is declare certain beliefs to be incompatible with your system of liberty and outlaw the practice of those beliefs. Certainly, this is the paradox of a (true) liberal society. The downfall of freedom comes because of the freedom to promote anti-liberalism.
The best way to keep evil men from wielding great power is to not create great power in the first place.