On March 6, 1820, President James Monroe signed the Missouri Compromise admitting Maine and Missouri as our nation’s 23rd and 24th states. Later that year, the 1820 Census found that our nation’s population was 9,638,458, including 298,335 people in Maine and 66,586 in Missouri.

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Soon after the United States purchased more than 828,000 square miles of land west of the Mississippi River from France in 1803, settlers began moving into the territory. In 1810, U.S. marshals conducted the first census in the Louisiana Purchase territory counting 19,783 people. By 1820, the number had grown to 66,557. As the territory's population grew, so too did its inhabitants desire for statehood. Should a new state be carved from the Missouri Territory, residents would enjoy rights guaranteed to all states by the 10th Amendment to the U.S. Constitution, including the ability to elect delegates to the U.S. Senate and House of Representatives.

With the nation bitterly divided between slave and free states, the admittance of a new state like Missouri would tip the balance of power between the North and South in Congress. Legislators of many free states believed slavery was immoral and argued that slavery should not spread north of the 36º 30' latitude line, which included Missouri. The slave states argued that Missouri's residents, not the federal government, should decide if the new state would be a free or slave state. Furthermore, slave state legislators and the territory's inhabitants argued that Missouri was already home to a sizeable slave population. So many southern farmers had moved to Missouri with their slaves that the number of enslaved people in the territory had more than tripled from 3,011 in 1810 to 10,222 by 1820.

On January 8, 1818, a delegation representing the Missouri Territory presented its petition requesting statehood to Speaker of the House Henry Clay. With representatives of 11 free and 11 slave states in Congress, the Missouri Territory's request sparked debate between the two factions, both of which feared the impact that tipping the balance of power in Congress could have on the nation.

The debate over Missouri's statehood grew even more heated when Rep. James Tallmadge Jr. of New York proposed admitting Missouri as a slave state only if its enslaved population was gradually emancipated. Tallmadge suggested that the importation of slaves into Missouri should be prohibited and all enslaved children born after Missouri's statehood should be freed at age 25. Slave states balked at the free state representative's proposal. They argued that it was an unconstitutional restriction of Missouri's petition for statehood and an attempt by the free states to use the federal government to abolish slavery. The Tallmadge amendment passed the House but failed in the Senate.

Hope for Missouri's admittance to the Union was reinvigorated thanks to the growth of the secession movement in Massachusetts' northern territory known as the District of Maine. On July 26, 1819, more than 70 percent of voters in the District of Maine approved seceding from Massachusetts. Delegates approved Maine's state constitution during the constitutional convention in Portland, ME, and petitioned the Congress for statehood. When Maine's petition for statehood arrived in Washington, DC, an opportunity for a compromise arose that could break the deadlock over Missouri's statehood. The Senate amended Missouri's statehood legislation to permit slavery in that state if Maine were admitted as a free state, thus preserving the balance of power between the North and South.

Despite using Maine's statehood petition as leverage, Congress remained deadlocked over the expansion of slavery. Sen. Jesse Thomas of Illinois—a free state —proposed a compromise that allowed slavery in Missouri, but prohibited its expansion into the remaining Louisiana Purchase territory north of the 36º 30' latitude line. The proposal gained support from some Tallmadge amendment proponents. After countless hours of backroom negotiations and arm-twisting by President James Monroe and Speaker of the House Clay, the U.S. House of Representatives passed the bill on March 2, 1820. The next day, slave state representatives moved to reconsider the vote. Clay declared the motion out of order until routine business had been completed. He then quietly signed the bill and sent it to the Senate. By the time slave state representatives again asked for the vote's reconsideration, it was too late. The Senate had already passed the legislation. President Monroe signed the Missouri Compromise into law on March 6, 1820.

In the decades that followed the Missouri Compromise of 1820, Congress maintained a precarious balance of power by admitting an equal number of free and slave states to the Union. For example, Congress admitted Arkansas to the Union as a slave state on June 15, 1836, followed soon after by the free state of Michigan on January 26, 1837. On May 30, 1854, the Kansas-Nebraska Act repealed the Missouri Compromise by allowing territories' residents to decide if they were to be free or slave states, regardless of their location above or below the 36º 30' latitude line. Passage of the act led to violent conflict between pro- and anti-slavery settlers in the Kansas Territory foreshadowing the bitter divisions between the North and South that would soon lead to the American Civil War (1861-1865).

You can learn more about the Missouri Compromise of 1820 using Census Bureau data and records. For example:

• Rep. Clay was instrumental in writing the Missouri Compromise of 1820. Clay was born in Hanover County, Virginia, in 1777. After moving to Lexington, KY, he served in the Kentucky State House of Representatives, U.S. Senate, U.S. House of Representatives, and as U.S. Secretary of State. As Speaker of House, Clay won northern and southern votes for the Missouri Compromise with an agreement to maintain a balance of power that admitted Maine as a free state and Missouri as a slave state. His mediation of political crises like the Missouri Compromise, the Nullification Crisis (1832-33), and the Compromise of 1850 earned him the nickname the "Great Compromiser." 
• Along with Henry Clay, James Tallmadge and Jesse B. Thomas also played critical roles in the passage of the Missouri Compromise. Tallmadge was born in Stanford, NY, in 1778. He served a single term as a member of the U.S. House of Representatives (1817-1819) representing Nassau County, New York. In 1819, his "Tallmadge Amendment" would have restricted—and eventually ended—slavery following Missouri's statehood. The amendment passed the U.S. House of Representatives thanks to overwhelming northern support, but failed in the U.S. Senate.  Jesse B. Thomas was born in Shepherdstown, VA (present-day West Virginia) in 1777. He moved to the Illinois territory after President Monroe appointed him as district judge there in 1809.  As senator from Illinois (1818-1829), Thomas proposed allowing slavery in Missouri, but excluding it from all other parts of the Louisiana Purchase territory north of Missouri's southern border. The Senate passed Thomas' provision, but it was voted down in the House of Representatives by northern states intent on admitting Missouri as a free state. Clay eventually mollified both sides and passed the Missouri Compromise of 1820 by admitting Missouri as a slave state, maintaining the balance of power in Congress by admitting Maine as a free state, and agreeing to Thomas' provision that slavery would be prohibited north of the 36° 30' parallel.
• President Monroe signed the Missouri Compromise into law five months before U.S. marshals began conducting the 1820 Census.  The census found that the nation's population grew 33.1 percent from 7,239,881 in 1810 to 9,638,453 in 1820.
• Missouri became the nation's 24th state on August 10, 1821. In accordance with the Missouri Compromise, slavery was legal in Missouri whereas it had been against the law in Massachusetts and Maine since 1783. In 1810, U.S. marshals counted 3,011 slaves in the Louisiana Territory (including the area that became the state of Missouri). Five months after the Missouri Compromise became law in 1820, the number of enslaved people in Missouri had grown to 10,222. Missouri's enslaved population was 25,091 in 1830; 58,240 in 1840; and 87,422 in 1850.  The 1860 Census—the last before the abolition of slavery—recorded 114,931 enslaved people living in Missouri.
• When Maine was admitted to the Union on March 15, 1820, Portland was the state's capital. In 1820, the Portland had a population of 8,581. Augusta became the state capital in 1827, but legislators continued to meet in Portland until builders finished construction of government buildings in Augusta in 1837. Three years after legislators moved to Augusta, the 1840 Census counted 5,314 people living in that city. In 2023, Portland was Maine's largest city with an estimated population of 69,104, while the state capital of Augusta was home to 19,102.
• Congress passed the Missouri Compromise to protect the political balance of power between the less populous, rural southern states and the more populous, industrial northern states. With a population of 123,706 in 1820, New York City, NY, was the nation's largest urban place, followed by Philadelphia, PA (63,802) and Baltimore, MD (62,738). New Orleans, LA (27,176), and Charleston, SC (24,780) were the only southern cities in the nation's 10 largest urban places. In fact, New Orleans, Charleston, and Richmond, VA (12,067) were the only southern cities that reported populations over 10,000 in the 1820 Census.
• During the period between the Missouri Compromise of 1820 and the Compromise of 1850 (which admitted California as a free state, established governments of Utah and New Mexico, etc.), legislators sought to maintain an equal balance of free and slave states in the U.S. Senate.  States were admitted in pairs during this period with the admission of a free state followed soon after by the admission of a slave state or vice versa. For example, the slave state of Arkansas became the nation's 25th state on June 15, 1836, followed by the admission of the free state Michigan on January 26, 1837. During this period, the nation's total population grew from 9,638,453 in 1820 to 23,191,876 in 1850. During the same 30-year period, the population of enslaved people in the United States more than doubled, growing from 1,538,038 in 1820 to 3,204,313 in 1850.
• In 1854, President Franklin Pierce signed the Kansas-Nebraska Act repealing the Missouri Compromise of 1820 and allowing voters to decide if new territories would allow slavery. On March 6, 1857, the U.S. Supreme Court ruled that the Missouri Compromise was unconstitutional in its landmark Dred Scott v. Sandford ruling. Attorneys for Dred Scott argued that Scott—an enslaved man—was freed when his owners took him into the free state of Illinois and the Wisconsin Territory. The court ruled against Scott. Chief Justice Roger Taney wrote that people of African descent were not  citizens according to the U.S. Constitution, so they had no jurisdiction to have their cases heard before a federal court. Furthermore, the court struck down the Missouri Compromise of 1820 ruling that the creation of a boundary above which slavery was illegal was an unconstitutional exercise of congressional power depriving slaveowners of their property rights. The Supreme Court hoped this ruling would calm tension between the North and South, but it only polarized the nation further. The Dred Scott Decision played a significant role in the 1860 presidential election in which Abraham Lincoln argued against the court's ruling and  vowed that he would not permit slavery to spread to new states or territories.

President Theodore Roosevelt signed legislation creating a permanent Census Office within the Department of the Interior on March 6, 1902.

On July 1, 1902, the U.S. Census Bureau officially opened its doors under the leadership of William Rush Merriam.

In 1903, the Census Office was moved to the newly created Department of Commerce and Labor. It remained within Commerce when Commerce and Labor split into separate departments in 1913.