The Dred Scott decision invalidated the idea of popular sovereignty promoted by Steven Douglas.
The Dred Scott decision basically said that slavery would be legal in all states of the Union. Living in a free state did not make a slave free. A Slave owner could bring his slaves with him into a free state that had voted to outlaw slavery. The slaves would remain his slaves even thought that was against the will of the people as expressed in popular sovereignty.
Douglas and the more moderate leaders in the North lost support due to the Dred Scott decision. This combined with the split in the democratic party allowed Abraham Lincoln and the Republicans to win the White House.
