Mainly the right of secession (the title of Bassett's pamphlet) but interwoven with that the principle that just government derives its powers from and rests on the consent of the governed. See here:Principles? What "principles"
Smith discusses some of the very few abolitionists who defended the right of southern states to secede from the Union.
I don't agree with the author's conclusions about the history, by the way, but he nonetheless presents some interesting history.