During the so-called Era of Good Feelings under President James Monroe, Missouri applied for statehood in 1819 with roughly 10,000 enslaved people already brought into the territory, largely to grow hemp. New York Representative James Tallmadge Jr. proposed an amendment barring further enslaved people from entering Missouri and gradually freeing enslaved children at age 25. The North held the House 105 to 81, even with the three-fifths clause inflating Southern seats, but the Senate stood at 11 free states and 11 slave states, so Southern members used it to block the restriction.

Northerners cited the Republican Guarantee Clause and the Northwest Ordinance; Southerners pointed to the 1803 Louisiana Purchase Treaty and argued a diffusion theory that spreading slavery west would ease emancipation. After the 15th Congress deadlocked, Maine’s request for statehood opened a path: Senator Jesse B. Thomas proposed barring slavery north of the 36°30′ parallel, and Speaker Henry Clay split the linked bills to move them through the House. A second compromise later in 1820 used ambiguous wording over Missouri’s attempt to bar free Black residents.

• Northern critics blamed the three-fifths clause for the Virginia dynasty of Southern presidents.
• Former Federalist Rufus King was among the Northern restrictionists.
• Some Southern members warned of disunion, and some Northern members answered that they would accept civil war.
• Monroe and Jefferson suspected the crisis was a Federalist plot to revive a strong central government.
• Jefferson compared the line to a fire bell in the night, and it held until the Kansas-Nebraska Act and Dred Scott.