The backstory of the Dred Scott v. Sanford, was essentially that Dred Scott, a slave of Sanford, sued for his freedom, since his master moved to a free territory, he should be automatically freed, per the requirements of the land. However, the Supreme Court ruled that he still had to stay as a slave, as he was Sanford's property, and property is not limited simply because of the type of territory they were in. Essentially, as long as the slave was bought at a slave state, the owner can keep him/her regardless if they move to a free state or not.
The effect of this on the North was public outrage, as they believed that the south would overstep and become too powerful as compared to the North. After all, the ability to move slaves into newly 'created' western territories, coupled with the historical 3/5th Compromise, resulted in large significant portion which may become slave territory, thereby tipping the balance of governmental power in favor of the south. The second effect of the Dred Scott case was the doubling down of the efforts towards the Underground Railroad, as well as the extension to British Canada, which was a free state. In essence, it was not enough for slaves to escape to a northern territory, as they can still be caught and returned to their masters under the Dred Scott v. Sanford ruling. However, if they were to escape to Canada, the US would then have no authority or obligation in which to capture the escapee.
Essentially, the North protested the case ruling, and put in more effort in ensuring that escaping slaves would stay free, by extending the underground railroad into Canada.
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