Who spoke out against the acquisition of the southwest following the mexican-american war

Dispute over whether any Mexican Territory that America won during the Mexican War should be free or a slave territory. David Wilmot introduced an amendment stating that any territory acquired from Mexico should be free. This amendment passed the White House twice, but failed to ever pass in Senate. The "Wilmot Proviso" became a symbol of how intense dispute over slavery was in the U.S., Bill that would ban slavery in the territories obtained after the War with Mexico.