He used a complex system of legal and natural law arguments in order to show that the clauses usually interpreted as supporting slavery did not in fact support it and that several clauses of the Constitution prohibited the states from establishing slavery. Spooner's arguments were cited by other pro-Constitution abolitionists such as Gerrit Smith and the Liberty Party, whose twenty-second plank of the 1849 platform praised Spooner's book "The Unconstitutionality of Slavery". Frederick Douglass, originally a Garrisonian disunionist, later came to accept the pro-Constitution position and cited Spooner's arguments to explain his change of mind. From the publication of this book until 1861, Spooner actively campaigned against slavery. He published subsequent pamphlets on jury nullification and other legal defenses for escaped slaves and offered his legal services to fugitives, often free of charge. In the late 1850s, copies of his book were distributed to members of Congress causing some debate over their contents. Even Senator Albert G. Brown of Mississippi, a slavery proponent, praised the argument's intellectual rigor and conceded it was the most formidable legal challenge he had seen from the abolitionists to date. In 1858, Spooner circulated a "Plan for the Abolition of Slavery", calling for the use of guerrilla warfare against slaveholders by black slaves and non-slaveholding free Southerners, with aid from Northern abolitionists. Spooner also "conspir[ed] with John Brown to promote a servile insurrection in the South" and participated in an aborted plot to free Brown after his capture following the failed raid on Harper's Ferry, Virginia (now part of the state of West Virginia). Although he had advocated the use of violence to abolish slavery, Spooner denounced the Republicans' use of violence to prevent the Southern states from seceding during the American Civil War. He published several letters and pamphlets about the war, arguing that the Republican objective was not to eradicate slavery, but rather to preserve the Union by force. He blamed the bloodshed on Republican political leaders such as Secretary of State William H. Seward and Senator Charles Sumner, who often criticized slavery yet would not attack it on a constitutional basis and who pursued military policies seen as vengeful and abusive. While denouncing the institution of slavery, Spooner recognized the right of the Confederate States of America to secede as the manifestation of government by consent, a constitutional and legal principle fundamental to Spooner's philosophy. In contrast, the Northern states were trying to deny the Southerners that right through military force.