So basically, I think I have a good grasp of the demographics who supported Democrats (farmers, urban immigrants) and Whigs (merchants, professionals, upper class) in the North (if I’m wrong here please let me know) but what about the South? On one hand, it seems clear that the Whigs and their unionist successors were more dominant in the Upper South, and assuredly among the non-slave owning, poor Appalachian types of that area, while Democrats dominated the Deep South. However, I was looking at some county level maps from the 1840s, and it appeared that the Whigs were mainly winning in the Black Belt area of wealthy slave owners, while Democrats were strong in upland areas like northern Alabama. Is one (or both) of these interpretations inaccurate, and is there some thing I’m missing here?
Can Democrats dominate the Deep South and the Whigs still dominate wealthy slaveholding counties in the 1840 election cycle? Yes, indeed.
I can only speak fully on one aspect of your question, which is Alabama in the 1840 cycle. At this point in history, Whigs are the party of economic security, and thus the party of wealthy slaveholding Alabamians. The nation is in post 1937 Panic and the Bank of Alabama is struggling with insolvency. Everyone's minds are one thing: the economic security of the country. And the more a man's economic situation is tied to commerce and the outside world, the more concerned he is going to be with the economic situation of the nation, federal relief, establishment of the national bank, etc.
Your post distinguishes between upper South and lower South in terms of values, but in this 1840 election cycle Alabamians are voting strictly in terms of economic interests. Any areas of the state in which Alabamians would be engaging in commerce outside the state would be pretty solid Whig. Coastal counties and plantation and production counties, etc. Both Whigs and Democrats were pro slavery in full, and while the Whig party did attack Democrats as soft on slavery in the 1840 election cycle, their efforts were rewarded with the Democrats' white basis voting plan (Alabama's answer to the 3/5ths compromise, limiting state level votes to whites only). Alabama was not voting according to pro or anti slavery sentiment in terms of human rights at this time. Remember, Democrats are the encumbents, the establishment party. Unless someone has a reason to shift to Whig, such as fear that the national economic crisis will affect their own incomes, they will stay firmly Democrat.