Reading a recent thread about the causes of the Civil War it appeared to me that at least by the 1860s the Democratic Party was what we would consider today conservative, almost reactionary, while the Republicans were more liberal, even progressive. Today, however, it's the complete opposite.
Provided my impression is correct, when did that shift took place and what were the causes?
While Republicans were more racially progressive(if that term can be applied to either part in the 19th century) than the Democrats at the outbreak of the Civil War, they were still more economically conservative than their Democratic counterparts.
The Democratic party became more liberal in the 1960's, when northern Democrats begin to support the Civil Rights Movement at the federal level, losing them support in the south, while winning support in the more "progressive" northern states. While this article starts at 1896, it looks at a few different potential causes for the role-reversal, though it seems the issue of race was the single biggest cause.
http://www.stat.columbia.edu/~gelman/research/published/reversal2.pdf