Every trained historian throws up in his mouth a little when somebody says we shouldn't judge historical figures by modern values - because they all reject that notion as fundamentally stupid. For one thing - none of these values are modern. The only thing that change from then to now regarding these values is how popular they are.
Go to any past event that modern society would reject - and you will find people alive THEN who actively campaigned against this event, the modern value was always already there.

And your point is really stupid because even if we IGNORE the great realignment of the civil rights era there isn't a single person anywhere in the democratic party who was in the democratic party in 1856. We damnwell SHOULD judge the democrat officials and voters of 1856 by the values of the democratic officials and voters in 2017 however.

And those who still holds those views SHOULD be hated by TODAY's demcoratic party. In case you were wondering -they are known as 'republicans'