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The Slave Power (often called the "Slaveocracy") was a hostile term used in the United States 1840-1865 to denounce the political power of the slaveholding class in the South. The argument was that this small group of rich men had seized control of their own states and was trying to take over the national government in illegitimate fashion to use it to expand and protect slavery.
The issue was not the treatment of slaves. It was fear of slave oligarchs. Men and women could differ on scores of issues, hate blacks or like them, denounce slavery as a sin or promise to guarantee its protection in the Deep South, and still attack the "slaveocracy." It mattered not where one stood on the other issues. One could still hate the slavemasters with a passion. 
The term was popularized by antislavery intellectuals such as John Gorham Palfrey, Josiah Quincy, Horace Bushnell, James Shepherd Pike, and, after the war, Horace Greeley and Henry Wilson. They showed through a combination of emotive argument and hard statistical data that the South had long held a disproportionate level of power within the nation. Did the slave power really exist? Millions in the North thought so, and acted upon it. However the notion was ridiculed by Southerners at the time, and rejected as false by historians of the 1920s and 1930s, who stressed that the South was internally divided 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 beliefs of Northerners.
Southern power derived from a combination of factors. The "three-fifths clause," (counting 100 slaves as 60 people for seats in the House and thus for electoral votes) gave the South additional representation at the national level. Parity in the Senate was critical, whereby a new slave state was admitted in tandem with a new free state. Regional unity across party lines was essential on key votes. In the Democratic party, the a presidential candidate had to carry the national convention by a two-thirds vote to get nominated. It was also essential for some northerners--"Doughfaces"--to collaborate with the South proved crucial, as in the debates surrounding the three-fifths clause itself in 1787, the Missouri Compromise of 1820, the gag rule in the House (1836-1844), and the wider subject of the Wilmot Proviso and slavery expansion in the Southwest after the Mexican war of 1846-48. However, the North was adding population--and House seats--much faster than the South, so the handwriting was on the wall. With the implacable Republicans gaining every year, the secession option became more and more attractive to the South. Secession was suicidal, as some leaders realized--and as John Quincy Adams had long prophesied. Secession, argued James Henry Hammond of South Carolina, reminded him of "the Japanese who when insulted rip open their own bowels." And yet when secession came in 1860 Hammond followed. Richards concludes, "It was men like Hammond who finally destroyed the Slave Power. Thanks to their leading the South out of the Union, seventy-two years of slaveholder domination came to an end." 
Threat to republicanism
The problem posed by slavery, it was argued by opponents of the Slave Power was not so much the mistreatment of slaves (a theme that abolitionists emphasized), but rather the political threat to American republicanism, and more generally to American standards of liberty. The Free Soil Party first raised this warning in 1848, arguing that the annexation of Texas as a slave state was a terrible mistake. Their strong rhetoric became a central theme in the new anti-slavery Republican party, especially in the election of 1856.
The Republican argument was that slavery was economically inefficient, compared to free labor, and was a deterrent to the long-term modernization of America. Worse, said the Republicans, the Slave Power was systematically seizing control of the White House, the Congress, and the Supreme Court. Senator and governor Salmon P. Chase of Ohio was an articulate enemy of the Slave Power, as was Senator Charles Sumner of Massachusetts and Republican party leader Abraham Lincoln of Illinois. The Southerns replied that they were committed to democracy and republicanism, and that assaults on their "peculiar institution" (slavery) was an illegitimate effort to make them second class citizens. By 1850 they talked of secession.
In his "House Divided" speech of June 1858, Abraham Lincoln charged that Senator Stephen A. Douglas, President James Buchanan, his predecessor, Franklin Pierce, and Chief Justice Roger Taney were all part of a plot to nationalize slavery, as proven by the Court's Dred Scott decision of 1857.
Other Republicans pointed to the violence in Kansas, the brutal assault on Senator Sumner, attacks upon the abolitionist press, and efforts to take over Cuba as new slave territory, as evidence that the Slave Power was violent, aggressive, and expansive.
The only solution, Republicans insisted, was a new commitment to free labor, and a deliberate effort to stop any more territorial expansion of slavery. Northern Democrats answered that it was all an exaggeration and that the Republicans were paranoid. Their Southern colleagues spoke of secession, arguing that John Brown's raid on the federal arsenal at Harpers Ferry in 1859 proved that the Republicans were ready to attack their region and destroy their way of life.
In congratulating President-elect Lincoln in 1860, Salmon P. Chase exclaimed, "The object of my wishes and labors for nineteen years is accomplished in the overthrow of the Slave Power", adding that the way was now clear "for the establishment of the policy of Freedom" — something that would come only after four destructive years of Civil War.
Blue (2006) explores the motives and actions of those who played supportive but not central roles in antislavery politics—those who undertook the humdrum work of organizing local parties, holding conventions, editing newspapers, and generally animating and agitating the discussion of issues related to slavery. They were a small but critical number of voices who, beginning in the late 1830s, battled the institution of slavery through political activism. In the face of great odds and powerful opposition, activists insisted that emancipation and racial equality could only be achieved through the political process. Representative activists include: Alvan Stewart, a Liberty party organizer from New York; John Greenleaf Whittier, a Massachusetts poet, journalist, and Liberty activist; Charles Henry Langston, an Ohio African American educator; Owen Lovejoy, a congressman from Illinois, whose brother was killed by a pro-slavery mob; Sherman Booth, a journalist and Liberty organizer in Wisconsin; Jane Grey Swisshelm, a journalist in Pennsylvania and Minnesota; George W. Julian, a congressman from Indiana; David Wilmot, a congressman from Pennsylvania whose Wilmot proviso tried to stop the expansion of slavery in the Southwest; Benjamin Wade and Edward Wade, a senator and a congressman, respectively, from Ohio; and Jessie Benton Frémont of Missouri and California, wife of the Republican 1856 presidential nominee John C. Frémont.
Impact of Democratic Free Soilers
The Democrats who rallied to Martin Van Buren's "Free Soil Party" in 1848 have been studied by Earle (2003). Their views on race occupied a wide spectrum, but they were able to fashion new and vital arguments against slavery and its expansion based on the Jacksonian Democracy's long-standing commitment to egalitarianism and hostility to centralized power. Linking their antislavery stance to a land-reform agenda that pressed for free land for poor settlers -- realized by the Homestead Law of 1862--in addition to land free of slavery, Free Soil Democrats forced major political realignments in New York, New Hampshire, Massachusetts, and Ohio. Democratic politicians such as Wilmot, Marcus Morton, John Parker Hale, and even former president Van Buren were transformed into antislavery leaders. Many entered the new Republican party after 1854, bringing along Jacksonian ideas about property and political equality, helping transform antislavery from a struggling crusade into a mass political movement that came to power in 1860.
- "Between the slave power and states' rights there was no necessary connection. The slave power, when in control, was a centralizing influence, and all the most considerable encroachments on states' rights were its acts. The acquisition and admission of Louisiana; the Embargo; the War of 1812; the annexation of Texas "by joint resolution" [rather than treaty]; the war with Mexico, declared by the mere announcement of President Polk; the Fugitive Slave Law; the Dred Scott decision — all triumphs of the slave power — did far more than either tariffs or internal improvements, which in their origin were also southern measures, to destroy the very memory of states' rights as they existed in 1789. Whenever a question arose of extending or protecting slavery, the slaveholders became friends of centralized power, and used that dangerous weapon with a kind of frenzy. Slavery in fact required centralization in order to maintain and protect itself, but it required to control the centralized machine; it needed despotic principles of government, but it needed them exclusively for its own use. Thus, in truth, states' rights were the protection of the free states, and as a matter of fact, during the domination of the slave power, Massachusetts appealed to this protecting principle as often and almost as loudly as South Carolina."
- Leonard L. Richards, Slave Power: The Free North and Southern Domination, 1780-1860 (2000) p. 3)
- See Chauncey S. Boucher, "In Re That Aggressive Slavocracy," The Mississippi Valley Historical Review Vol. 8, No. 1/2 (Jun., 1921), pp. 13-79; Craven (1936)
- Garry Wills, "Negro President": Jefferson and the Slave Power, (2005)
- Most Doughfaces were Jacksonian Democrats like Franklin Pierce and James Buchanan; few were Whigs.
- Richards (2000)
- Richards (2000) pp. 214-15.
- See Blue (2006)
- See Earle (2003)
- Henry Adams, John Randolph (1882) pp 178-79