More to the point, once the Democrat/leftist/socialists lost, how would they be punished for starting a civil war?
After the US Civil War, the Democrat South was effectively put under military rule, which became Republican rule though they were still a small minority in the South.
Reconstruction lasted for 14 years, and many of its elements survived it, either immediately, or after southern Democrats abused their restored liberty to abuse others, by the courts.
So the question becomes: if these leftists try another such scheme, how and for how long must they again have a boot on their back?
I can imagine NYC, Chicago, Los Angeles, San Francisco and some other cities put under federal law; and maybe even a few blue states.
We have voting lists & plenty of knowledge of political affiliations. In the aftermath of a civil war I would personally think stripping leftists of their citizenship & escorting them to the border would be the most simple thing. The next step would have to be re-education. Anyone who is a Democrat but did not fight on the wrong side would have to accept a period of time learning about US history & what is wrong with socialism/communism. Anyone who does not pass or argues with teacher would join the others.
Communism & socialism would have to be outlawed. We can no longer tolerate those who want to change a free country into a slave nation.