That's what the party that wished to keep it's majority control of Congress said. The reality is it could *NOT* expand, at least not anywhere else in the United States. It could have perhaps expanded into Mexico, but that would be an internal affair of Mexico, and not an affair of the United States.
But slavery could not expand into the West. There simply was no way to establish any kind of large scale plantations in the West. The climate and lack of water would have doomed any such adventure at least until the discovery of the Ogallala Aquifer and modern pumping equipment to make irrigation possible. Probably 60 years in the future, from 1860.
Slavery could have been adopted to minining.