After the war, he served as Illinois Attorney General. He was a prominent member of the Republican Party and, though he never had an elected job, he was nonetheless an active participant in politics. According to Robert Nisbet, Ingersoll was a "staunch Republican." His speech nominating James G. Blaine for the 1876 presidential election was unsuccessful, as Rutherford B. Hayes received the Republican nomination, but the speech itself, known as the "Plumed Knight" speech, was considered a model of political oratory. His opinions on slavery, woman's suffrage, and other issues of the time would sometimes become part of the mainstream, but his atheism/agnosticism effectively prevented him from ever pursuing or holding political offices higher than that of state attorney general. Illinois Republicans tried to persuade him to campaign for governor on the condition that Ingersoll conceal his agnosticism during the campaign, which he refused to do.