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The way it actually worked in practice is that when Congress set up territories, it would designate them as Free Soil or Slave Territories. Early in the nation, the area above the Ohio River was so designated as Free, while KY and Tennessee were slave territories. This managed to extend the dividing line of Free/Slave more or less westward and controversy was at a minimum. Slaveholders would move to a slave Territory, non-slaveholders to a free one, and when the state was set up there was little surprise to anyone. The residents would set up the constitution of the new state to perpetuate the existing arrangement.
This worked out well enough until two things happened - first the gain of considerable non-slaveholding area from Mexico led to some calls to keep this going (the Wilmot Proviso). The other destabilizing matter was that the land in this new SouthWest was not suited for high manpower farming via slaves anyway. With no room for expansion, the South started to think of itself as 'encircled'. The American universal equation of lack of territorial growth with stagnation hurt as well.
Upvote:0
Free and slave states were determined largely by climate. That is, land south of the Ohio River and the Mason-Dixon Line (between Pennsylvania and Maryland) was suitable for cash crops such as tobacco, cotton and sugar,and therefore slavery, while land north of it was more suitable for subsistence farming (free labor).
The other factor is that with slavery being so contentious, legislators tried to keep a reasonable balance of states. The 13 Colonies had seven free and six slave states (divided by the Mason-Dixon Line). Vermont was the eighth free state, and Kentucky and Tennessee the seventh and eighth slave state. As long as the "U.S." was just east of the Mississippi River, one could expect that Ohio, Indiana and Illinois would be free, while the future Missisippi, Alabama, and Florida would be slave.
The Louisiana Purchase changed all that. Because the territory is "funnel shaped" out of 12 future states, only Louisiana, Arkansas, and Missouri were likely slave states, the remainder probably free (with the possible exception of Kansas). The acquisition of Texas changed things back; Southerners thought that it could be broken up into "several" slave states to balance the excess Louisiana Territory free states.
In determining the status of "free" or slave states, the U.S. Congress by and large followed the will of the people in the state, although there were some attempts to "stuff" states (e.g. Kansas) with pro slavery people.
Upvote:15
This isn't quite right. Slavery was considered a commercial matter, and thus the only part of it Congress felt they could regulate was its interstate commerce aspect. IOW, slave trading. This is why the Constitution specifically forbid Congress banning slave trading until 1808. That was all the danger they felt Congress could pose directly to slavery.
So Congress felt they were powerless to regulate it within established states. However, what laws governed a territory prior to statehood were set by Congress, so slavery within territories was entirely within their power. Also, it was entirely up to Congress how to decide to let in new states, so they were perfectly within their rights to not allow entry of a state to the union if they didn't like what that state's constitution said about slavery.
The hole in this argument is that a Constitutional Amendment can regulate behavior within states. So Slavery can be banned that way. However, that requires two thirds of either states or their Senators.
So as long as at least one third of states stayed slave as states were added, slavery couldn't be banned in them.
But again, a majority vote in Congress is used to regulate territories and admit states. So in the long run, simply having a minority of slave states sending Senators to Congress makes slavery vulnerable. This is why the slave states felt that every new free state added to the union had to be balanced with at least one more slave state.