Discussion on answer by TitaniumTurtle: What are some plausible factors and societal changes that could result in a modern civil war in the United States?

Note that the U.S. Democratic party is "left" leaning only with respect to the U.S.-specific Overton window. In Europe it would be considered centrist at best, even center-right. It's nowhere near a Socialist party; from across the ocean it looks to be more like traditional European center-right parties such as the Conservative party of the U.K. or the Christian Democratic Union of Germany. AFAIK, there is no consensus in the U.S. Democratic party to introduce strict gun control, and no inclination towards nationalization of companies.

@Nosajimiki-ReinstateMonica: My point was that they are, in fact, very much not Communists or even Socialists, and thus they will not, in fact, implement any kind of Communist or Socialist policies which would trigger mass resistance. I'm not at all current with American politics, but doesn't the Democratic party actually have an iron grip on California? And didn't they actually live very well with a two-term Republican governor, Arnold Schwarzenegger? Isn't California one of the richest and most dynamic states, home of many flourishing companies?