No one was.
In the Election of 1860, the northern Democrats ran their own candidate for president, differing with their southern counteparts over the slavery issue.
During the war, northern Democrats again separated into factions, the War Democrats, who supported Lincoln's military policies; and the Copperheads/Peace Democrats, who opposed them.
Copperheads were mainly located in the Midwest, where many families had Southern roots and were agricultural types, hence their attitude.