Dunning was nominated by the Democratic party to run for Congress in 1856, but he declined primarily due to the fractured nature of the party at that time. He, along with a large part of the party, was at odds with its state leadership who were sympathetic to the south. In 1860 he did accept a nomination to be a delegate to the party's convention in Charleston, South Carolina, and the second convention held in Baltimore. In both convention he voted in favor of Stephen Douglas, and favored compromise on the slavery issue. When the American Civil War finally broke out in the next year, Dunning publicly declared himself for the Union and set to work raising companies of men for the war effort. In the end, he was personally responsible to the recruitment of nearly two full regiments of men.