I think it was less Southern nationalism than state loyalty, but pro-slavery ideology certainly played a substantial part, at least among the leaders of the political class. I disagree re racial anxiety - that is more or less a Northern thing, as most of us in the South have grown up side by side with black people - something that definitely didn't happen up North. (When I happened to be in Muskegon Michigan for a week back in the 70s, the only black person I saw was a one-legged water skier in a touring show.)
From Nat Turner to John Brown, there had been much fear that if White control slipped, the result would be murder or the destruction of civilization. The Democrats appealed to such racial fears in both the North and the South, and found plenty of adherents. Similar anxieties about Blacks taking over were still present in the 1950s and 1960s.
The other side of the coin is that White Southerners weren't afraid of Blacks as such, since they'd lived among them all their lives, so eventually after centuries, desegregation might be more successful and thoroughgoing in the South than in the North. That's because both races tended to live in the same neighborhoods, so residential segregation wasn't as much of an issue. But if we are talking about the past, we can't pass over its distinctive features.
And while Southern fears of Blacks as such were less, one can turn that around as well, and say that if Northerners had a slave class and a system of subjugation, they wouldn't have been so afraid of Blacks, either. When charges of hypocrisy come to dominate arguments, the charges can get tossed around forever without the argument or knowledge or agreement advancing. "You're not as good as you think" is something that depends more on how one thinks other people think about themselves than about actual circumstances.
In both the North and the South, some people ran ahead and drove events and passions toward conflict and others lagged behind. After the Civil War, many people accepted the "Robert E. Lee" view that most Confederate notables were sorrowing stoics with no enthusiasm for slavery, secession or war, who dutifully went with their states or at most, fought to defend their ideas of state's rights and liberty. But without passionate pro-slavery agitators, war would have been less likely.
If you look at DeBow's, the Charleston Mercury and other Lower South publications, you'll find much idealization of slavery and much enthusiasm for Southern nationhood. Probably only a minority thought that way, but it was an influential minority. You could draw a parallel between abolitionists in the North, and pro-slavery or Southern nationalist fire-eaters in the South. The importance of both groups outweighed their numbers, since they were able to drive the debate ever further from the mainstream. As the terms and limits of debate changed, moderates and centrists followed where the radicals led.