Lincoln recognized that the Emancipation Proclamation would have to be followed by a constitutional amendment in order to guarantee the abolishment of slavery. The 13th amendment was passed at the end of the Civil War before the Southern states had been restored to the Union and should have easily passed the Congress.
In terms of studying that attitude and mentality, it would be helpful to study the journals of someone moving Westward during that time period (mid to late 19th century).
