Slave Power was pejorative - but deserved, and accurate - term used by opponents of slavery in the U.S. to identify the corrupting influence that slavery had on the United States of America. The Republican Party made it a main theme of the 1850s, arguing that the slave owners had seized control of most of the national government and were using it to their own ends. The Republicans argued this violated Republicanism--that is, it violated basic American political values.
The "Slave Power" (often called the "Slaveocracy") was in common use in the Ante-Bellum era 1840-1860 as a pejorative to identify and 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, 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 few historians now disagree that it was a powerful factor in the beliefs of Northerners.
Slave 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, 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 on the floor of the Senate, 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 the John Brown raid of 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.
The Republicans believed that the Confederacy was created by and for the benefit of the Slave Power, and that destroying one involved destroying the other.
In August, 1861, he supported the first law attacking slavery, the Confiscation Act that said owners would forfeit any slaves they allowed to help the Confederate war effort. By December he was the first Congressional leader pushing for emancipation as a tool to weaken the rebellion. He called for total war on January 22, 1862:
"Let us not be deceived. Those who talk about peace in sixty days are shallow statesmen. The war will not end until the government shall more fully recognize the magnitude of the crisis; until they have discovered that this is an internecine war in which one party or the other must be reduced to hopeless feebleness and the power of further effort shall be utterly annihilated. It is a sad but true alternative. The South can never be reduced to that condition so long as the war is prosecuted on its present principles. The North with all its millions of people and its countless wealth can never conquer the South until a new mode of warfare is adopted. So long as these states are left the means of cultivating their fields through forced labor, you may expend the blood of thousands and billions of money year by year, without being any nearer the end, unless you reach it by your own submission and the ruin of the nation. Slavery gives the South a great advantage in time of war. They need not, and do not, withdraw a single hand from the cultivation of the soil. Every able-bodied white man can be spared for the army. The black man, without lifting a weapon, is the mainstay of the war. How, then, can the war be carried on so as to save the Union and constitutional liberty? Prejudices may be shocked, weak minds startled, weak nerves may tremble, but they must hear and adopt it. Universal emancipation must be proclaimed to all. Those who now furnish the means of war, but who are the natural enemies of slaveholders, must be made our allies. If the slaves no longer raised cotton and rice, tobacco and grain for the rebels, this war would cease in six months, even though the liberated slaves would not raise a hand against their masters. They would no longer produce the means by which they sustain the war."
One of the key reasons for Lincoln's announcement of the Emancipation proclamation in September, 1862, was to reassure the North that the destruction of the Slave Power was indeed a war goal.
In Reconstruction, one of the main objectives of the U.S. Congress, especially the Radical Republicans like Charles Sumner and Thaddeus Stevens was to guarantee that the Slavery Power was permanently dead. They decided the only way to achieve this was to give Freedmen (freed slaves) the right to vote, while stripping the vote away from leavers of the Confederacy. Despite widespread doubts in the North, in fact the white South unanimously agreed that slavery was dead and there were no efforts whatever to resurrect it.
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."
- Ashworth, John. "Free Labor, Wage Labor, and Slave Power: Republicanism and the Republican Party in the 1850s," in The Market Revolution in America: Social, Political and Religious Expressions, 1800-1880, edited by S. M. Stokes and S. Conway (1996), 128-46.
- Blue, Frederick J. No Taint of Compromise: Crusaders in Antislavery Politics (2004) excerpt and text search
- Boucher, Chauncey S. "In Re That Aggressive Slavocracy," The Mississippi Valley Historical Review Vol. 8, No. 1/2 (Jun., 1921), pp. 13-79 in JSTOR
- Craven, Avery. "Coming of the War Between the States An Interpretation," The Journal of Southern History, Vol. 2, No. 3 (1936), pp. 303-322; rejects notion of Slave Power in JSTOR
- Davis, David Brion. The Slave Power Conspiracy and the Paranoid Style (1969)
- Earle, Jonathan. Jacksonian Antislavery and the Politics of Free Soil, 1824-1854 (2004) excerpt and text search
- Foner, Eric. Free Soil, Free Labor, Free Men: The Ideology of the Republican Party Before the Civil War (1970)], esp pp. 73-102 online edition
- Gara, Larry . "Slavery and the Slave Power: A Crucial Distinction" Civil War History v15 (1969), pp 5-18
- Gienapp, William E. "The Republican Party and the Slave Power," in Robert H. Abzug and Stephen E. Maizlish, eds., New Perspectives on Race and Slavery in America (1986), pp 51-78
- McInerney, Daniel J. "'A State of Commerce': Market Power and Slave Power in Abolitionist Political Economy." Civil War History 1991 37(2): 101-119. Issn: 0009-8078
- Richards, Leonard L. Slave Power: The Free North and Southern Domination, 1780-1860 (2000), the standard scholarly history; excerpt and text search
- Trefousse, Hans L. Thaddeus Stevens: Nineteenth-Century Egalitarian (1997) online version
- Woodburn, James Albert. The Life of Thaddeus Stevens: A Study in American Political History, Especially in the Period of the Civil War and Reconstruction. (1913) online version
- Cairnes, John Elliott. The Slave Power: Its Character, Career, and Probable Designs (1862), British political tract online text of the second edition; complete text online
- Lowance Jr., Mason I. ed. House Divided: The Antebellum Slavery Debates in America, 1776-1865 (2003) excerpt and text search
- Thompson, C. Bradley ed. Anti-Slavery Political Writings, 1833-1860: A Reader (2003)
- Henry Wilson, History of the Rise and Fall of the Slave Power in America (in 3 volumes, 1872 & 1877) vol 2 online; vol 3 online
- The Slave Power speeches of abolitionist Theodore Parker, 1841-52
- Republican Party
- Democratic Party
- Third Party System
- American Civil War
- Abraham Lincoln
- Thaddeus Stevens
- Charles Sumner
- 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," 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.
- Woodburn 178-179
- See Blue (2006)
- See Earle (2003)
- Henry Adams, John Randolph (1882) pp 178-79