Antislavery as a principle entailed far more than simple limitations on the extent of the institution of slavery. However, because slavery was such a divisive national issue and largely handled on the state level instead of federally, many Northerners advocated for a policy of gradual and compensated emancipation as a means of placating proslavery Southerners. Historians distinguish between moderate antislavery reformers, or gradualists, who concentrated on stopping the spread of slavery, and radical abolitionists, or immediatists, whose demands for unconditional emancipation often merged with a concern for African-American civil rights. Nearly all Northern politicians, including Abraham Lincoln, rejected more radical abolitionists. Indeed, many Northern leaders married into slave-owning Southern families without any moral qualms, including Stephen Douglas (the Democratic nominee for president in 1860), John C. Fremont (the Republican nominee for president in 1856), and Ulysses S. Grant.

Gradual Abolition

An Act for the Gradual Abolition of Slavery, passed by the Pennsylvania legislature on March 1, 1780, was the first attempt by a government in the Western Hemisphere to begin the abolition of slavery. The Act prohibited further importation of slaves into the state, required Pennsylvania slaveholders to annually register their slaves (under pain of forfeiture for noncompliance, and manumission for the enslaved), and established that all children born in Pennsylvania were free persons regardless of the condition or race of their parents. Those enslaved in Pennsylvania before the 1780 law went into effect remained enslaved for life. In 1847, the Pennsylvania legislature passed another act freeing its slaves altogether.

Pennsylvania's "gradual abolition"—as opposed to Massachusetts's 1783 "instant abolition"—became a model for freeing slaves in other Northern states. Other states that enacted gradual abolition include New Hampshire (1783), Connecticut and Rhode Island (1784), New York (1799), and New Jersey (1804). With the exception of New Jersey, these states were not as conservative as Pennsylvania about slaveholders' property rights. Their gradual abolition laws freed future children of slaves at their birth, and all slaves after a certain date or period of years. New Jersey's gradual abolition law freed future children of slaves at birth, but those enslaved before the passage of the gradual abolition law remained enslaved for life. The December 6, 1865, ratification of the Thirteenth Amendment to the U.S. Constitution ended slavery in the United States, but New Jersey's legislature did not approve the Thirteenth Amendment until February 1866, two months after it had been ratified by a three-fourths majority of all states.

The Constitutionality of Slavery

In the early 1850s, the American abolitionist movement split into two camps over the issue of the U.S. Constitution. This issue arose in the late 1840s after the publication of The Unconstitutionality of Slavery by Lysander Spooner. The Garrisonians, led by William Lloyd Garrison and Wendell Phillips, publicly burned copies of the Constitution, calling it a pact with slavery, and demanded its abolition and replacement. Another camp, led by Lysander Spooner, Gerrit Smith, and eventually Frederick Douglass, considered the Constitution to be an antislavery document. Using an argument based upon Natural Law and a form of "social contract" theory, they said that slavery existed outside of the Constitution's scope of legitimate authority and therefore should be abolished. 

Activism

Many American abolitionists took an active role in opposing slavery by supporting the Underground Railroad. Though illegal under the Fugitive Slave Law of 1850, participants such as Harriet Tubman, Henry Highland Garnet, Alexander Crummell, Amos Noë Freeman, and others put themselves at risk to help slaves escape to freedom.

Other abolitionists spread word of the horrors of slavery in an attempt to win more supporters for their cause. A prominent example of this is Harriet Beecher Stowe, author of Uncle Tom’s Cabin, or Life Among the Lowly. Stowe, from a pious Connecticut family, had strong moral convictions that slavery as an institution was evil and unnatural. Her book depicted the harsh conditions in which slaves lived, the danger they were willing to place themselves in to escape, and the detrimental ways in which the institution of slavery effected slave owners. Uncle Tom’s Cabin was a success in the North, selling more than 300,000 copies in the first nine months of its publication, and more than a million copies by 1853. Nonetheless, it was met with protest and alarm in the South.

The abolitionist movement was strengthened by the activities of free African Americans, especially in the black church, who argued that biblical justifications for slavery contradicted the teachings of the New Testament. African-American activists often did not have access to wide audiences outside of the black community; however, they were tremendously influential to some white audiences who were already sympathetic to their cause. One such sympathetic abolitionist was William Lloyd Garrison. Garrison's efforts to recruit eloquent spokesmen from within the African-American community led him to Frederick Douglass, who was a prominent activist in his own right. Eventually, Douglass would publish his own widely distributed abolitionist newspaper, The North Star.

Proslavery riot

Elijah P. Lovejoy at Alton Illinois Riot 1837