The fight over whether or not to continue slavery in the United States was a major issue from the very early days of the Republic. In 1774, Benjamin Franklin, was one of the founding members of the Pennsylvania Society for Promoting the Abolition of Slavery. Franklin described slavery as "an atrocious debasement of human nature" at the Continental Congress. At the First Continental Congress it was agreed to discontinue the slave trade.
Thomas Jefferson, author of the Constitution, had an ambiguous relationship to slavery. Jefferson was a slave owner, yet he thought slavery was bad. He introduced a bill in the Virginia legislature that called for the gradual emancipation (freeing of) all the slaves in Virginia. In Jefferson's first draft of the Declaration of Independence he wrote: "He has waged cruel war against human nature itself, violating it's most sacred rights of life and liberty in the persons of a distant people who never offended him, captivating and carrying them into slavery in another hemisphere, or to incur miserable death in their transportation thither. This piratical warfare, the opprobrium of infidel powers, is the warfare of a Christian king of Great Britain. Determined to keep open a market where men should be bought and sold, he has prostituted his negative for suppressing every legislative attempt to prohibit or to restrain this execrable commerce. "
The delegates from the Deep South opposed these words. In a compromise to achieve their support, the words in the section were removed. Jefferson did succeed in keeping the following words: "All men are created equal" in the preamble- this was the first compromise relating to slavery.
In the years that followed, many of the Northern states passed laws that gradually freed their slaves. In 1780, the Massachusetts Supreme Court ruled slavery violated the Massachusetts Constitution. One of the last acts of the Congress that met under the Articles of Confederation was to pass the "Northwest Ordinance". This ordinance prohibited slavery in the new territories that were to become the states of Ohio, Indiana, Michigan, Illinois and Wisconsin.
In 1787, Jefferson continued his personal struggle with slavery, when he wrote in his notes on the State of Virginia: "and can the liberties of a nation be thought secure when we have removed their only firm basis, a conviction in the minds of the people that these liberties are the gift of God? That they are not to be violated but with his wrath? Indeed I tremble for my country when I reflect that God is just: that his justice cannot sleep forever... I think a change already perceptible, since the origin of the present revolution. The spirit of the master is abating, that of the slave rising from the dust, his condition mollifying, the way I hope preparing, under the auspices of heaven, for a total emancipation, and that this is disposed, in the order of events, to be with the consent of the masters, rather than by their extirpation. " When the framers of the Constitution met in Philadelphia, slavery was a major divide between the representatives of the northern states and those of the south. Clear compromises were made. It was agreed, as part of the Constitution, that when calculating the number of people in a state (enumeration) slaves would be counted as 3/5 of a person, giving the southern states, whose population consisted of up to 40% more power, based on individuals who had no rights.
The Constitution also explicitly blocked the new national government from outlawing slave trade before 1808. The Constitution also directly protected the rights of slave holders, by adding a clause protecting their rights: "No person held to service or labor in one state, under the laws thereof, escaping into another, shall, in consequence of any law or regulation therein, be discharged from such service or labor, but shall be delivered up on claim of the party to whom such service or labor may be due. "

Many of the framers of the Constitution were optimistic slavery would not endure. Roger Sherman stated at the Constitutional Convention: "the abolition of slavery seemed to be going on in the United States and that the good sense of the several states would probably by degrees complete it".
Despite the hope of the founders, slavery did not wither on the vine in the years following the Constitutional Convention. In fact, slavery become an ever more important factor in the Southern economy. The rising price of cotton and sugar on the world markets made growing these crops continually more profitable. In addition, with the Spanish in retreat, and the aftermath of the slave revolt in Haiti, the South had less competition for a plantation-based economy. Another major factor was the invention of the cotton gin. This great technological advance made the growing of cotton considerably more profitable. The growing of cotton soon became the lifeblood of the South. With the end of the War of 1812 trade with Great Britain rapidly expanded. Cotton exports to Great Britain doubled, between 1815 and 1820. Cotton exports doubled again in the next five years.
Finally, an unexpected development occurred. The end of the Atlantic slave trade, made slaves even more valuable. It became a simple issue of supply and demand. As no new slaves were arriving from Africa, the slaves in the southern states became all the more valuable. The only way to create more slaves was to breed them. The US expanded westward and there was need for slaves in the new territories. The only way to supply these slaves was from the original slave states. The export of slaves soon became a thriving business for the original slave states.
All of these changes had a profound change on the attitude of the South to slavery. In the early years of the republic Southerners had been defensive about slavery admitting it was a bad thing, but seeing no way to end it. But as slavery evolved, Southerners became more impassioned in their defense of slavery.