Read about the Commune in Paris in 1870—Communist defended the city for months before it fell. That’s how LA and SF would be defended. Millions would die. Fight for the New People’s Republic of California—or be shot by the Red Guards! It would rip the nation apart—but I don’t think it would last long—maybe a year.
During the period of the Paris Commune, many of the citizens were MUCH more self-sufficient than would prove to be the case today. So far as food goes, the Parisians of the time has a ready supply within the city, the horses which were then everywhere, and so far as fuel, they relied almost entirely on wood, which would have been harvested from the parks and streets overshadowed by the tree cover. Nobody today would be able to find enough animal life within large cities (unless they turned to cannibalism), or have the means to harvest the tree growth, as almost everything is powered by some source of electricity, or petroleum based fuel, and with the sophisticated means of waste removal now inoperative, would choke on their own garbage within a very few days, as the water flow stopped, the toilets overflowed, and the streets became impassible because of the mountains of rubbish. And this is to presume the populace remains quiescent.
There's no such commitment in LA. In SF, maybe, but it's easier than LA geographically.
New York is very vulnerable to cut-off, but there's a large working class population that might be glad to see the snobs and hipsters tossed out.