Listen to this history teacher for a second right here.

There is a concept in historical teaching to signify the positioning of political parties in the United States, I tend to call it "Northern Platform" and "Southern Platform". The Northern Platform is a lot more urban, pro-big government and pro-industry, the Southern Platform is a lot more rural, pro-state government and pro-agriculture. Civil Rights are not that urgent an issue in the two platforms, even though Southern Platform parties tend to be more socially conservative and conscious about social reform.

That way, you can for example classify Democrat-Republicans as the Southern Platform and the Whig Party as the Northern Platform.

Now tell me, which party is the Northern Platform one right now? The Democrats, as evidenced by their performance in urban areas, are more in step with the urban (/northern) platform. They are pro-big government, pro-social reform, pro-healthcare, all that. And you will probably agree that they are the more "liberal" party in terms of social and political issues.

Which was it back in the 1860s? Exactly, the Republicans. They were the young party coming after the collapse of the Whigs, they were against slavery, they were for more government involvement in economic affairs.

And with the Southern Platform, it is exactly reversed obviously, since no party except for these two ever had a significant support over a longer period of time since the 1860s.