I hope you're correct. The most similar situation of which I'm aware, the Finnish civil war that followed that nation's independence in the same timeframe as the Russian Communist takeover pitted Finnish Communista and their allied *Red* forces against the Finnish nationalist and Republican *whites*. Following some four months of bloodletting, the Red survivors found themselves either in the detention camps of the Whites, lined up against the nearest logpile motti and shot without ceremony or accountability, or fleeing to their Soviet neighbor...where a few years later, a suspicious Stalin had most of them shot or sent to the Gulag; some estimates believe Stalin killed more Finnish Communists than the White forces did.
But in any event, the casualties, when most of the smoke had cleared and the bloodstained Finnish snow had melted, was between one and three percent of the Finnish population.
Extrapolated to the figures of the 2000 US census population of 281,421,906, that'd be around 3 million bodies laying around at a minimum, a significant public health problem. And it may well come to that.
I think it's doubtful that once started such a second American civil war would cease in a couple of weeks, and probably would not conclude within 4 months. But I'd expect that between one to three percent of the population would no longer be present, and possibly as high as three to five percent if it continued much beyond half a year. Once begin such conflicts are horrible, indeed.