Hi, you seem to be putting very modern political labels on 19th century politics, and I'm a bit confused what you're asking about - I'm not sure that the term 'alt-left' would have made any sense at all to the Democrats of 1850. But if you're basically asking why the Democrats and Republicans have 'swapped sides' since the Civil War, there's some good posts by /u/Samuel_Gompers answering the questions Can someone address a brief history of Democrats vs Republicans, specifically the change in Dems from the early 1900s being against civil rights to a more progressive party in the 50/60s leading much social change in the U.S. and Why was there such a huge shift in the core viewpoints of the Republican party in the past 100 years?.