Perhaps the single strongest unifying factor of pre-Civil War southern whites was their ___.

The correct answer is that the strongest unifying factor of pre-Civil war southern whites was their perception of white racial superiority

Explanation:

White southerns main reason to fight the civil war was because they believed and wanted to keep the rigts to continue to enslave African Americans as they thought it was their right and it was protected by the constitution.

The whites believed they were superior than African Americans for a long time which ended up leading to 400 years of slavery.