The thing is highly left and populist doesn't mean liberal. A highly populist South is easy to come up with with the right POD during reconstruction, of course this would be populist in favor to the working whites. You could also see a highly fiscally conservative North appear with the right changes in the gilded age. And with attitudes stirred the right way you can make exaggerate these tendencies if you have a reason for the North and South wanting to be as different from each other as possible.
I want to assume the liberalism in the OP means social liberalism, which is much more harder for it to come about. But not impossible if you find ways to alter immigration patterns. Keep the North Puritan and the South open to immigration and urbanization.
Lets say for some reason the eerie canal never gets built, the importance of New York as the port of access to the America interior is weakened. Baltimore rises as an alternative, as does New Orleans; NYC would still very important but the other two would be picking up much more slack. In the case of NO, it could result in much heavier traffic through the Mississippi River than OTL, and urbanization along its banks. Have Birmingham develop as an industrial center earlier, and your adding traffic through the Tennessee River. As a result of this Southern economy grows and diversifies. Attitudes towards keeping a plantation style economy change, and by the mid 1800s more Southerners are open to the idea of ending slavery. Some investment and you can see shipyards in the gulf growing in importance as well.
On top of this add some Latin-American / caribbean immigration into the South through NO, and Mobile AL, to keep this new industry going. Ibero/Caribbean attitudes of race, which while still racist can be much more fluid, influence Southern attitudes. As a result Irish and Italian immigration ends in the south later on. (This will then change attitudes regarding alcohol and other politics).
Then shift political attitudes to delay the Civil War. End of the day only, the very deep south and a political alliance made of the plantation owners rebel. So much so that it would not be called a Civil War but a slaveholder's rebellion. A good amount of Southerners stay loyal, and the rebels are easily singled out afterwards. The loyal Southerners try to build an identity away from the rebelling slaveholders and politicians. Post-rebellion southern politics are dominated by reformers and liberals.
After the rebellion have a useful reconstruction that allows blacks to have a strong economic base (40 acres and a mule type of deal). This allows them to stay in the South. All of this results in a much more populous and economically dynamic south.
Meanwhile in the north trade in the great lakes has suffered, anywhere outside the coast is mostly the breadbasket "king corn" and "king wheat". Have the silver/long depression in the late 1800s be more sever. Like in OTL it will affect mostly the farming and mining mid-west, but here it kick starts a populist/conservative political attitude in the Great Lakes states and midwest, which outside the coast cities really screws up any progressive development.
The difference between the North and South, will really be seen in the early 20th century during the prohibition era. The North, particularly the protestant and rural west bans the sale of alcohol. Meanwhile in the South, particularly the coast - which harbors some hedonistic tendencies at this time - alcohol is seen as part of life. In fact burbon, in the upper south, and rum, in the coastal south, is seen as an integral of their cultural identity. Consumption goes up only to contrast northern attitudes.