All of slavery in this country was on the democrat side.
Northern democrats supported slavery. Southern democrats supported slavery.
Republicans were opposed to slavery.
https://www.youtube.com/watch?v=b-Kb3p56qA8
The Amendment was named after Ohio Republican Corwin, and pushed very strongly by New York Republican Seward.
It was in fact passed by the Congress and sent to the states for ratification, and three Northern states actually did ratify it.
At the founding of this country all 13 Original Colonies had slaves. Made no difference what political party they adhered to.
“The Confederate soldier did not give a damn about slavery, all he cared about was stopping the yankees from invading and destroying their land.” - Shelby Foote