McKim worked for several years as an agent of the American Anti-Slavery Society, lecturing in many different areas of Pennsylvania and the adjacent counties of New Jersey and Delaware. It was not an easy life. Traveling accommodations were primitive, and public opinion was always unenthusiastic, often downright hostile to his cause. His meetings were sometimes interrupted by rowdies with fife and drum or barking hounds. In some places pickets paraded with signs attacking him as a tool of the British and an advocate of racial amalgamation. Tomatoes, eggs, garbage, and even stones were thrown at him. In some communities he was unable to hold meetings at all. When violence loomed at one of his lectures in Gettysburg, Thaddeus Stevens intervened to calm the crowd and restore order, threatening personally to prosecute the offenders “to the very door of the penitentiary.”
Despite the hardships involved, the campaign which McKim and other agents waged was crowned with considerable success. About a hundred antislavery societies were started in Pennsylvania within a period of two or three years. The culmination of this crusade was the organization of the Pennsylvania State Anti-Slavery Society on January 31 and February 1-3, 1837. This was the society with which McKim was to labor for over twenty years. In 1838 he paid a memorable visit to Washington, D.C., where he observed conditions in the slave marts and came away with the conviction that “no man can fully appreciate the horrors of American slavery.” Later that year he formally abandoned the Presbyterian ministry, announcing that he could no longer give his assent to “the doctrine of a vicarious atonement.”