In 1860 they were Democrats, allied to Southern slavers to rule over Washington, DC, since the election of 1800.
Thrown out of power in 1860 by rural Republican voters, Southern Democrats declared secession & war against the United States.
Their Northern Big-City Democrat allies either supported the slavers or opposed them only half-heartedly, i.e., George McClellan.
After the Civil War, Democrats North & South quickly reunited and have been waging some form of warfare against the United States ever since.
Why is that so hard for you to grasp?
If what you are saying was true, the tariffs would have been 13% as the Confederate states made them, and the Congress would have voted to allow slavery in the territories.
Clearly none of that happened, so clearly you are wrong. Washington DC was controlled by Big City Northern Liberals, and it is still controlled by Big City Northern Liberals.