What is the Constitutional process for secession? There is none given in the Constitution.
Both the arguments of the Confederate states and the arguments of Lincoln relied of two different sets of moral sentiments and assumptions (not law) they believed the Constitution relied on, but the clear legal language for neither was placed in the Constitution.
Yes, I think the Confederate states had the better philosophical argument, even though I reject their cause (to preserve slavery) but no I don’t think they were on any better Constitutional grounds than Lincoln; unfortunately due to the founders unwillingness to place legal language for secession in the Constitution.
It's given in the Declaration of Independence, and i'm pretty sure everyone involved in the Constitutional convention was familiar with the process. :)