During the period before the American Civil War, Parker remained loyal to the Democratic Party and advocated a moderate course in the hope that concessions on the slavery issue would avoid bloodshed. In 1861, he was the permanent chairman of the state Democratic convention. Once the war started, he supported the Union, but argued against what he saw as the excesses of the Lincoln administration. In 1864, he successfully argued the case of "Palin v. Murray" in Greene County, obtaining a judgment for the plaintiff on the grounds of false imprisonment by federal authorities. The case was later moved to the federal courts, where it was decided in Palin's favor by the U.S. Supreme Court in 1869.