The abolitionists would, he was certain, use that power (and, ironically, this is pretty much what Abraham Lincoln ended up doing):
"[T]hey will search that paper [the Constitution], and see if they have power of manumission," said Henry. "And have they not, sir? Have they not power to provide for the general defence and welfare? May they not think that these call for the abolition of slavery? May they not pronounce all slaves free, and will they not be warranted by that power?
"This is no ambiguous implication or logical deduction. The paper speaks to the point: they have the power in clear, unequivocal terms, and will clearly and certainly exercise it."