What is Slave Power?

Legal Definition
The terms "Slave Power" and "slaveocracy" were used by antislavery campaigners in the U.S. in the 1840s and 1850s, in reference to what they saw as the disproportionate political power held by slave owners in the federal government. The argument was that this small group of rich slave owners had seized political control of their own states and were trying to take over the federal government in an illegitimate fashion in order to expand and protect slavery. The argument was widely used by the Republican Party that formed in 1854–55 to oppose the expansion of slavery.

The main issue expressed by the phrase was distrust of the political power of the slave-owning class. Such distrust was shared by many who were not abolitionists; those who were motivated more by a possible threat to the political balance or the impossibility of competing with unwaged slave labor, than by concern over the treatment of slaves. Those who differed on many other issues (such as hating blacks or liking them, denouncing slavery as a sin or promising to guarantee its protection in the Deep South) could unite to attack the "slaveocracy." The "Free Soil" element emphasized that rich slave owners would move into new territory, use their cash to buy up all the good lands, then use their slaves to work the lands, leaving little opportunity room for free farmers. By 1854 the Free Soil Party had largely merged into the new Republican party.

The term was popularized by antislavery writers such as John Gorham Palfrey, Josiah Quincy III, Horace Bushnell, James Shepherd Pike, and Horace Greeley. Politicians who emphasized the theme included John Quincy Adams, Henry Wilson and William Pitt Fessenden. Abraham Lincoln used the concept after 1854 but not the term. They showed through a combination of emotive argument and hard statistical data that the South had long held a disproportionate level of power in the United States. Historian Allan Nevins contends that "nearly all groups ... steadily substituted emotion for reason. ... Fear fed hatred, and hatred fed fear."

The existence of a "Slave Power" was dismissed by Southerners at the time, and rejected as false by many historians of the 1920s and 1930s, who stressed the internal divisions in the South before 1850. The idea that the Slave Power existed has partly come back at the hands of neoabolitionist historians since 1970, and there is no doubt that it was a powerful factor in the Northern anti-slavery belief system. It was standard rhetoric for all factions of the Republican party.
-- Wikipedia