Maybe. But the election of Lincoln was the first reversal they had suffered after many more significant victories. Meanwhile, Lincoln seemed less than eager to end slavery at the outset of his term, and I would think that they could likely have simply waited him out in anticipation of victories down the road rather than taking rash action and hoping for an absolute victory in the present. If they had succeeded in annexing Cuba and other territories, they would have further cemented their Congressional advantage, and their Electoral College advantage.
Sure seems like an overreaction to me.
In addition to deciding that a freed slave couldn't be considered a citizen, the Supreme Court had indicated in Dredd Scott that Congress had no authority to restrict slavery in the territories.
See Section III:
http://www.ourdocuments.gov/doc.php?doc=29&page=transcript
Essentially, the Court ruled that Congress could only legislate for a territory in the same way that it legislates over the whole United States, therefore it couldn't directly make laws which would normally be the domain of a state government. The upshot of this is that while unorganized, a territory must be open to slavery. It essentially abolished the concept of a free territory unless that territory had a government that banned slavery itself. So whatever Lincoln did, the new territories would likely have come in as slave territories, and then slave states, unless at some point along the way, the political will to change that and dispossess at least some of their citizens could be mustered.
In addition, Dredd Scott ruled that a slave could be brought into a "free" state without gaining his or her freedom. Since that person has no means of access to the court, this undermines the very concept of a free state. Perhaps the free states could ban their own citizens from owning slaves themselves, but there was no way for them to avoid men and women being brought among them in chains, and by virtue of the Fugitive Slave Act, they were actively conscripted to ensure that none would escape.
So, it was in the context of the collapse
favoring the Slave Power of many compromises and carefully balanced powers that Lincoln was swept into office. The states of the coming Confederacy had been on the verge of winning by political and legal means all that they wanted, but in their unwillingness to accept an election that they considered averse, they lost everything.