Is there any 20th-century POD that could have led to a large-scale shooting war within the United States, potentially leading to partition? The only remotely plausible one I can imagine is the situation in Reds! with a stronger socialist movement and an uprising during the Depression.
There certainly could have been larger-scale insurgencies emerging out of a fractured Civil Rights Movement, but I don't think they could have become serious enough to be termed a second Civil War.
Obviously anything after the early 70s is pretty much ASB.