In 1812, the "Lower Louisiana" was admitted to the Union as a state, "Upper Louisiana" was organized into the Missouri Territory. By 1818, Missouri had reached the point that its citizens petitioned for admission themselves. Since the institution of slavery had been an established element in Missouri since French colonial times, this would have created a situation where the U.S. Senate would have a majority of Southern slaveholding states. That prospect was extremely unwelcome in the North.
In addition to the economic question of slavery, there was a moral issue. Many in the north felt that the federal government should not allow slavery in its new territories and should move along a path to eventually destroy it. The House of Representatives put forward an amendment to the admission of Missouri that would prohibit the introduction of slaves into Missouri and freeing the children of slaves at the age of 25. Passed by the House in February, 1819, the Tallmadge Amendment was defended in the Senate by Rufus King of New York, who argued that the Constitution allowed the federal government to determine the conditions under which states would be admitted, and the Northwest Ordinance of 1787 provided the precedent for restricting or forbidding slavery. Southern opposition in the Senate, however, doomed the Tallmadge Amendment to defeat. The Senate passed the bill admitting Missouri without the amendment, but it was rejected by the House, pushing the controversy into 1820. Later in the year, Maine held a convention and requested admission as a "free" state. The Great Compromiser, Henry Clay, proposed the following elements of a sectional compromise: