Earlier, as Governor of Virgina, a state which allowed slaveholding, Monroe had to address the issue when, on October 15, 1799, some slave traders attempted to transport a group of slaves from Southampton, Virginia to Georgia. The enslaved men revolted and killed the slave traders. In response, the authorities killed ten of the men involved in the revolt on the spot without the benefit of trial. Five others were tried without the benefit of a jury. Four were convicted and the fifth entered a plea. His punishment was to be flogged and branded. As Governor, Monroe postponed their executions, ostensibly to check their identities. In the end he granted a pardon to one. Two of these men were later hung, while the other died in jail. Some historians credit Monroe with at least restoring the requirement of civil protection for enslaved persons accused of crimes for which they could be sentenced to death for capital crimes.
When Monroe was Governor of Virginia in 1800, a plot was discovered in which hundreds of enslaved persons from Virginia intended to kidnap Monroe, capture Richmond, and negotiate for their freedom. Due to a storm on August 30, they were unable to attack. This event is known as Gabriel's slave conspiracy. Monroe called out the militia, and so-called "slave patrols" captured some of the enslaved persons allegedly involved in the plot. They were given trials (though those trials as lacking the same procedural fairness given to accused persons who were white.) Monroe used his authority to pardon and sell some of these enslaved persons instead of hanging them. Despite this, between 26 and 35 of those accused were executed.
Monroe himself was a slave owner and owned dozens of enslaved persons. He took some of them to serve him when he resided at the White House from 1817 to 1825, a custom of other slaveholding presidents. There was no domestic staff provided for the presidents at that time.
The end of opposition parties led to the end of party discipline and the rise of internal factional animosities, which were often sectionally based. Rather than producing the political harmony that Monroe had hoped for, rivalries among Jeffersonian Republicans would eventually lead to the election of Monroe's successor being selected by the House of Representatives, and four years after that it led to the rise of Jacksonian Democracy.
The land added to the United States in the Louisiana Purchase would result in an issue that divided those in Monroe's party as well as the nation as a whole. Prior to its purchase in 1803, the governments of Spain and France had permitted slavery in the region. In 1812, the state of Louisiana became the first state to come out of the Louisiana Purchase. It had entered the Union as a slave state. In the years following the War of 1812, the region then known as the Missouri Territory, experienced rapid settlement, in large part by led by slaveholding planters. Most of these southern planters brought their enslaved persons into the territory and by 1820 about 15% of the a total population 67,000 were made up of those enslaved. As the population of Missouri territory reached the threshold that would qualify it for statehood, legislation was put before Congress empowering territorial residents to select convention delegates and draft a state constitution and apply for statehood, presumably as a slave state.
When the Missouri statehood bill was opened for debate in the House of Representative on February 13, 1819, it appeared at first that the matter would proceed routinely. In the course of these proceedings, however, Representative James Tallmadge Jr. of New York injected controversy by proposing the following amendments:
"Provided, that the further introduction of slavery or involuntary servitude be prohibited, except for the punishment of crimes, whereof the party shall have been fully convicted; and that all children born within the said State will be executed after the admission thereof into the Union, shall be free at the age of twenty-five years."
Tallmadge was an opponent of slavery and had played a leading role in accelerating the emancipation of all remaining enslaved persons in his state in 1817. His fellow New York Republican, Congressman John W. Taylor had previously proposed similar slave restrictions on Arkansas territory in the House, but his motion had failed 89-87. The amendment exposed the divisions among Jeffersonian Republicans over the question of slavery. Northern Jeffersonian Republicans allied with the few remaining Federalists while Southern Jeffersonians united in support of the expansion of slavery. Southerners united as a section, taking the position that the free states were not allowed to meddle in the affairs of the slaveholders. They accused the northern abolitionists as wanting to incite rebellion among the slave populations, something they saw as a grave threat to their security. Northern Jeffersonian Republicans referred to the Declaration of Independence, citing its promise that "all men are created equal."
Southerners were also afraid of a potential loss of power. Article One, Section Two of the US Constitution provided for legislative representation in those states where residents owned slaves in what was known as the three-fifths clause or the "federal ratio", under which three-fifths (60%) of the slave population was numerically added to the free population in determining the number of Congressional districts per state and the number of delegates to the Electoral College. Tallmadge and his supporters disliked the three-fifths clause because it had translated into an imbalance of power for the South.
On February 16, 1819, the House Committee of the Whole voted to link Tallmadge's provisions with the Missouri statehood bill, approving the move 79-67. Following this vote, debate resumed on Tallmadge's provisions. The enabling bill passed in the House by a margin of 87 to 76, with 22 abstentions. It then was put before the Senate, where both failed to pass.
Southerners in Congress viewed this as a threat to their sovereignty and to their "peculiar institution" of slavery. During the following session (1819–1820), the House passed a similar bill for the admission of Missouri as a slave state. Complicating matters was the fact that in December of 1819, Alabama had been admitted as a slave state, making the number of slave and free states equal. At the same time, there was a bill passed in the House on January 3, 1820, admitting Maine as a free state.
In a move that would contribute to his reputation as "the Great Compromiser", Senator Henry Clay of Kentucky proposed that Maine be admitted to the Union, with an amendment enabling the people of Missouri to form a state constitution. Before the bill was returned to the House, a second amendment was adopted on the motion of Jesse B. Thomas of Illinois, excluding slavery from the Louisiana Territory north of the parallel 36°30′ north (the southern boundary of Missouri), except within the limits of the proposed state of Missouri.
The vote in the Senate was 24 for the compromise, to 20 against. The amendment and the bill passed in the Senate on February 17 and February 18, 1820. The House then approved the Senate compromise amendment, on a vote of 90 to 87. The House then approved the whole bill, 134 to 42 on March 5, 1820. The bills were signed into law by President Monroe on March 6. The question of the final admission of Missouri came up during the session of 1820–1821. Through the influence Henry Clay, an act for the admission of Missouri as a slave state was finally passed.
Like other Presidents, Monroe expressed the opinion that slavery was morally wrong, but did so after his term in office. As president of Virginia's constitutional convention in the fall of 1829, Monroe reiterated his belief that slavery was a blight. He said in a speech "What was the origin of our slave population? The evil commenced when we were in our Colonial state, but acts were passed by our Colonial Legislature, prohibiting the importation, of more slaves, into the Colony. These were rejected by the Crown." At the convention, Monroe made his final public statement on slavery, proposing that Virginia emancipate and deport its bondsmen with "the aid of the Union."
Monroe was part of the African Colonization Society formed in 1816, which included members like Henry Clay and Andrew Jackson. These men were not abolitionists, but they did find common ground with some abolitionists who supported the notion of colonization, i.e. relocating former slaves to other countries. This group helped send several thousand freed slaves to Africa between 1820 and 1840. The concern slaveholders like Monroe and Jackson had was to prevent free blacks from influencing slaves to rebel in southern states. With about $100,000 in Federal grant money, the organization also bought land for those people in what is today Liberia. The capital of Liberia was named Monrovia after James Monroe. It is the only foreign capital to be named after an American President.