In 1860 virtually all Northerners, including Republicans, were content to allow slavery in the South.
All northerners understood that Southern slavery was the price of Union, that opposing slavery meant Southern secession.
What Northerners adamantly opposed was expanding slavery into their own states via the Supreme Court's Dred-Scott decision, or into Western territories which didn't want it.
Northern Democrats like President James Buchanan from Pennsylvania did everything they could to address Southern concerns about slavery.
ZULU: "Not all Southerners supported slavery."
Huge areas of western Virginia, Maryland & North Carolina, plus eastern Tennessee & Kentucky opposed slavery and the Confederacy and supplied troops for the Union Army.
They made a significant difference in the course of the Civil War.
ZULU: "The American Flag flew over a slave nation longer than any confederate flag."
Even though southern Founders like Washington, Jefferson and Madison understood that slavery was wrong and should eventually be abolished, other slave-owners made the institution of slavery a pre-condition to establishing the United States under its 1787 Constitution.
Bottom line: without slavery there would have been no United States.
And two separate countries would never have fought a Civil War to abolish slavery.
Dred-Scott did not have anything to do with expanding slavery in to northern states.