The Wilmot Proviso was a proposal to prohibit slavery in the territory acquired by the United States at the conclusion of the Mexican War.

In 1846, David Wilmot a Democratic member of the United States House of Representatives from Pennsylvania, proposed the Wilmot Proviso. He attached the proviso to an appropriations bill to pay Mexico for land that the United States had seized as a result of the Mexican War. The Wilmot Proviso would have prevented slavery's expansion into any of this new territory. The House of Representatives approved the appropriations bill and the proviso on August 8, 1846, but the Senate adjourned before it could debate the bill. The House adopted the bill and the proviso in its next session. On February 1, 1847, the Senate approved the bill but rejected the proviso. As a result, the proviso never went into effect.

The proviso passed the House of Representatives because a majority of the representatives came from the North. Under the United States Constitution, each state received representatives based on that state's population. The North had more people than the South.

In the Senate, there were the same number of slave and free states. Each state was entitled to two senators. When senators from the North and South voted along regional lines, a bill could not be approved. Northern and southern states intentionally tried to maintain the balance between slave and free states. As long as neither side had an advantage in the Senate, a bill could not be sent to the president to sign that would favor one side or the other.

The Wilmot Proviso further divided the North and the South over the issue of slavery. Many Southerners believed that slavery should be legal everywhere in the United States. A growing number of Northerners, including many Ohioans, opposed slavery's expansion. Some of these Northerners opposed slavery on moral grounds, arguing that African Americans were human beings. Other people feared economic competition from slave owners.