As a non american this seems prettt weird. Looking at todays parties, you would have thought that the Democrats would be anti slavery and the Republicans pro slavery.
Has anything changed within the years?
Hi, there's room for more input on this, but fyi you'll find information to get started here
Many Northern democrats opposed the expansion of slavery into the western territories, after all if they didn't oppose slave expansion the Democratic convention wouldn't have split in 1860 and Lincoln may not have been elected President. While democrats were historically more supportive of slavery than the Republicans and their whig predecessors, the election of 1860 largely followed sectional and not party lines