President Polk's alternative to the Wilmot Proviso was to fall back on the Missouri Compromise, an 1820 law that permitted Missouri to be admitted as a slave state with Maine as a free state.

Polk recommended extending the line of the Missouri Compromise west to the Pacific Ocean. That would have allowed slavery below the 36' 30' latitude line (present-day southern California, New Mexico, and Arizona) and forbid it above (present-day Washington, Oregon, Idaho, Utah, Nevada, and the northern half of California). Was slavery ultimately allowed in the new territory?