 In politics, he began as a Democrat, joined the Republican Party in 1856, and in 1860 again supported the Republican ticket.In 1861, he joined the Union Army as an aide-de-camp with the rank of major and served on the staff of General John C. Frémont, but at the close of the Missouri campaign Dorsheimer returned to civil life, and published a series of articles in the "Atlantic Monthly" entitled "Frémont's Hundred Days in Missouri.