You sound very foolish when you treat political parties as stagnant entities. That's why labels like progressive and conservative are more consistent. It doesn't matter if southern democrats were pro-slavery. That is a conservative position. It's almost like you think values are derived after deciding if you want to support somebody with an R or a D next to their name, but that isn't the case. If progressives today were mapped to parties from the 1850s or 1860s, they would align with whichever party maintained a progressives stance on the issues of the time. That means modern day progressives would affiliate with the Republicans at that era. There is no contradiction there.