The Missouri Compromise (March 3, 1820) was a United States federal legislation that stopped northern attempts to forever prohibit slavery's expansion by admitting Missouri as a slave state and Maine as a free state in exchange for legislation which prohibited slavery in the remaining Louisiana Purchase lands north of the 3630 parallel. The 16th United States Congress passed the legislation on March 3, 1820, and President James Monroe signed it on March 6, 1820.Earlier, in February 1819, Representative James Tallmadge Jr., a Democratic-Republican (Jeffersonian Republican) from New York, had submitted two amendments to Missouri's request for statehood that included restrictions on slavery. Southerners objected to any bill that imposed federal restrictions on slavery and believed that it was a state issue, as settled by the Constitution. However, with the Senate evenly split at the opening of the debates, both sections possessing 11 states, the admission of Missouri as a slave state would give the South an advantage. Northern critics including Federalists and Democratic-Republicans objected to the expansion of slavery into the Louisiana Purchase territory on the Constitutional inequalities of the three-fifths rule, which conferred Southern representation in the federal government derived from a state's slave population.

Jeffersonian Republicans in the North ardently maintained that a strict interpretation of the Constitution required that Congress act to limit the spread of slavery on egalitarian grounds. "[Northern] Republicans rooted their antislavery arguments, not on expediency, but in egalitarian morality." "The Constitution [said northern Jeffersonians], strictly interpreted, gave the sons of the founding generation the legal tools to hasten [the] removal [of slavery], including the refusal to admit additional slave states."When free-soil Maine offered its petition for statehood, the Senate quickly linked the Maine and Missouri bills, making Maine admission a condition for Missouri entering the Union as a slave state. Senator Jesse B. Thomas of Illinois added a compromise proviso that excluded slavery from all remaining lands of the Louisiana Purchase north of the 36 30' parallel. The combined measures passed the Senate, only to be voted down in the House by Northern representatives who held out for a free Missouri. Speaker of the House Henry Clay of Kentucky, in a desperate bid to break the deadlock, divided the Senate bills. Clay and his pro-compromise allies succeeded in pressuring half of the anti-restrictionist House Southerners to submit to the passage of the Thomas proviso and maneuvered a number of restrictionist House northerners to acquiesce in supporting Missouri as a slave state. The Missouri question in the 15th Congress ended in stalemate on March 4, 1819, the House sustaining its northern antislavery position and the Senate blocking a slavery restricted statehood.

The Missouri Compromise was very controversial, and many worried that the country had become lawfully divided along sectional lines. The KansasNebraska Act effectively repealed the bill in 1854, and the Supreme Court declared it unconstitutional in Dred Scott v. Sandford (1857), both of which increased tensions over slavery and contributed to the American Civil War. The compromise both delayed the Civil War and sowed its seeds; Thomas Jefferson writing contemporaneously predicted the line it had drawn would someday tear the Union apart. 40 years later, the North and South would split closely along the 3630 parallel and fight for four bloody years.

The United States Congress is the legislature of the federal government of the United States. It is bicameral, being composed of a lower body, the House of Representatives, and an upper body, the Senate. The Congress meets in the United States Capitol in Washington, D.C. Both senators and representatives are chosen through direct election, though vacancies in the Senate may be filled by a governor's appointment. Congress has 535 voting members: 100 senators and 435 representatives. The vice president of the United States has a vote in the Senate only when senators are evenly divided. The House of Representatives has six non-voting members.The sitting of a Congress is for a two-year term, at present, beginning every other January. Elections are held every even-numbered year on Election Day. The members of the House of Representatives are elected for the two-year term of a Congress. The Reapportionment Act of 1929 establishes that they be elected in single-member constituencies or districts by first-past-the-post and that Congressional districts be apportioned to states by population every ten years using the United States Census results, provided that each state has at least one Congressional representative. Each senator is elected at-large in their state for a six-year term, with terms staggered, so every two years approximately one-third of the Senate is up for election. Each state, regardless of population or size, has two senators, so currently, there are 100 senators for the 50 states.

Article One of the United States Constitution requires that members of Congress must be at least 25 years old (House) or at least 30 years old (Senate), have been a citizen of the United States for seven (House) or nine (Senate) years, and be an inhabitant of the state which they represent. Members in both chambers may stand for re-election an unlimited number of times.

The Congress was created by the Constitution of the United States and first met in 1789, replacing in its legislative function the Congress of the Confederation. Although not legally mandated, in practice since the 19th century, Congress members are typically affiliated with one of the two major parties, the Democratic Party or the Republican Party, and only rarely with a third party or independents affiliated with no party. In the case of the latter, the lack of affiliation with a political party does not mean that such members are unable to caucus with members of the political parties. Members can also switch parties at any time, although this is quite uncommon.