Complex sentence: The proposal, however, did not succeed in Congress because of opposition from anti-slavery Congressmen because many people in the area were pro-slavery with business connections in southern states, and this new territory lay below the old Missouri Compromise line of demarcation between slave and free states.
Split into simpler sentences: The proposal, however, did not succeed in Congress because of opposition from anti-slavery Congressmen. Many people in the area were pro-slavery, with business connections in southern states, and this new territory lay below the old Missouri Compromise line of demarcation between slave and free states.