Let's Talk about where Slavery is at the moment !
So as you may or may not know, the international slave trade was given a sunset provision as part of the constitution, with the slave trade meant to end by 1808. By the time of the election of 1804, only South Carolina still allowed the legal importation of slaves, nearly entirely from Africa but some from the Caribbean as well. This isn't to say they slave trade stopped - slaves were still taken from South Carolina to other states or were smuggled up through either the unorganized Mississippi Territory or Spanish Florida but demand was genuinely drying up.
In general by this time it seemed that slavery was going to sort of wean itself out of existence anyway. Virginia Maryland and North Carolina found tobacco production on the scale that required slave labor was impractical given how rough tobacco production was on Virginia's soil. The only true cash crops were sugar and cotton, neither of which grew well in Maryland or Virginia or even most parts of North Carolina.
Georgia and South Carolina however had vibrant sugar and cotton production and remained good markets for slaves but even with that there was a large excess of slaves in the United States and the question of what to do with all of the slaves that were now in surplus was being kicked around. The Cotton gin, invented during the Washington admin and popularized during the Adams had changed the calculation however. Prior to its invention cotton production was an onerous process that took a lot of time and production was limited by the long process of 'ginning' the cotton from the seeds which limited the scope of cotton growing plantations. With the invention of a machine that did the ginning for you, cotton production became much easier and cotton production exploded in the Carolinas, Georgia and the Mississippi territory.
By 1804, South Carolina and Georgia were more or less at capacity and trade on the Mississippi river was too unsure to make expanding new plantations into Mississippi worthwhile. This is what made the Louisiana purchase so tantalizing. Not only did it open new land for plantations in Mississippi and Louisiana but it meant new markets for Virginia and Maryland slave owners who began to pride themselves on their slave breeding practices, tracing bloodlines and achievements not entirely unlike horsebreeding.
Unchecked the Louisiana Purchase is sure to allow for an explosion in plantation production and a renewal/deepening of slave culture - something which has northern farmers nervous about their ability to compete