Im not going to deny that Slavery was an important issue for the Southern cause. However, I still view the issue as one POLITICAL in nature. Slavery just happened to be the cause they were protecting.
The Southern states never had a majority in the legislature, and because of this they could be bullied by the northern states in what was becoming a strong central government, contrary to what the intent of our republic was at first outset.
Slavery, being what they viewed as necessary to protect the success of the southern economy, was at stake because they didn't have the votes in the legislature to prevent the North from passing nearly any bill they wished. Therefore, the economy of the southern states was viewed to be in jeopardy, and held hostage by a northern majority. Similarly, in the mid 1750s, England voted to increase taxes on the colonies, effecting their economy, without adequate representation for them either.....see where I am going with this?
The south seceded because they virtually had no say in the central government, a central government getting stronger and stronger (and yet relatively weak compared to today's) where they had not the ability to stand up for themselves in the legislature, and opted for self governance instead.
It just so happened the catalyst for all this was Slavery....... which, when looking back for some can easily be turned into the arguments we hear today, without taking any of the above into consideration.