Popular Sovereignty
In the heat of the Wilmot Proviso debate, many southern lawmakers began to question the right of Congress to determine the status of slavery in any territory. According to John Calhoun, the territories belonged to all the states. Why should a citizen of one state be denied the right to take his property, including slaves, into territory owned by all? This line of reasoning began to dominate the southern argument. The Congress had a precedent for outlawing slavery in territories. It had done so in the Old Northwest with the passing of the Northwest Ordinance in 1787. The Missouri Compromise also had banned slavery above the 36º30' latitude lines. But times were different.
http://www.ushistory.org/us/30b.asp
