"[I can] scarcely conceive of a cause of sufficient power to divide this Union, unless a belief in the slaveholding States that it is the intention of the other states gradually to undermine their property in their slaves and that a disunion is the only means to avert the evil. Should so dangerous a mode of believing once take root, no one can calculate the consequences."
Letter of August 12, 1820, to Gallaway in the Markoe Papers in 'Library of Congress'
This idea was conceved fifteen years before the anti-pro slavery crisis "began" in 1835. It is easy to see that Calhoun's primary concern was how to split the Union, not save slavery. Calhoun was constantly trying to "force the issue", a favorite phrase of his.