This is a question that often stumps people, and the very fact that Republicans, by their own admission, couldn't and wouldn't abolish slavery directly is often seized on as "proof" that the war was actually not over slavery. Of course, the war was over slavery, but if Republicans weren't a threat to it, why secede in the first place? In fact, Republicans had a comprehensive anti-slavery program that Southerners perceived as a threat so big that secession was justified, a move that was profoundly ironic for it brought about the very revolution they were hoping to avoid sooner and more radical that it could have been otherwise. The issue has, in my view, four different components: 1) what Republicans said they would do, based on their understanding of constitutional and political limits; 2) what Republicans actually would do, how far were they willing and able to push their anti-slavery convictions; 3) what pro-slavery Southerners thought Republicans would do, and their perceptions of the possible effects of these policies; and, 4) what pro-slavery people thought should be done in the face of this perceived threat, and what the results of these countermeasures would be. Let's analyze all these points, shall we?

The Republican Party was alarming to pro-slavery people because it was a party built on the conviction that slavery was morally wrong. It was, furthermore, a sectional party that drew all its strength from the North, with nil or marginal support in most of the South. Republicans boldly said that slavery was morally wrong and should be ended by political action. But, how was this to be done? As you say, ending slavery required a constitutional amendment. It was unfeasible, and no one suggested this seriously as a solution because it was simply not politically possible. Indeed, abolitionists were more likely to say that the entire constitution was hopelessly pro-slavery and should be scrapped because under it the Federal government had simply no power whatsoever to interfere with slavery. All Republicans, except perhaps some very radical abolitionists, subscribed to this "Federal consensus". Slavery, they believed, could only be abolished by the states themselves. In the view of most Republicans, slavery was a decrepit old system, so inferior to Northern free labor that it would eventually collapse and be peacefully and constitutionally abolished in every state. Because the enslaved had no incentives to work, because plantation culture depleted the fields, and because slavery denigrated White labor and encouraged feudal relations that stifled progress and freedom, there was simply no way slavery would survive. The natural march of progress would result in abolition, just as it had happened in all states north of the Mason-Dixon line. Why, then, was slavery still alive in the South in the 1850's, and abolition so far away? Because of the Slave Power.

The "Slave Power" was an idea that occupied a very important place in the minds of Northerners, and it was probably the chief cause of their dislike of slavery, rather than the plight of the enslaved. According to it, slavery had been dying, something the wise Founders intended and expected. But a cabal of wealthy slaveholders banded together to hijack the institutions of government and extend unnatural protection and help to the hated institution. Seizing control over the Southern states and the Federal government, the Slave Power artificially kept slavery alive, subverting both human progress and the true will of the Southern people. Republicans pointed to the fact that most presidents had been slaveholders or their advocates, to the disproportionate Southern influence in the Congress and the Supreme Court, to the many pro-slavery measures taken. The events of the 1850's lent special credence to the Slave Power conspiracy, with Buchanan's blatant efforts in favor of slavery, the Dred Scott decision, the Kansas-Nebraska Act, and other such incidents that seemed to show that Republicans were right, that slaveholders were holding the Federal government hostage for their own ends. This harmed White men as well, because it meant that the progress of all the nation was stopped and that Southern states were free to tramp on the rights of Northern states, as seen in the Fugitive Slave Act. The solution, they said, was voting the Republican ticket, which would overthrow the Slave Power and start a new policy that would place slavery on the path of ultimate extinction.

But, what was this policy? If Republicans had already admitted they couldn't abolish slavery or interfere with it, what could they do? Some have taken this to mean that Republicans had no plan, that they wouldn't do anything at all against slavery. In truth, Republicans had a comprehensive program of anti-slavery action within the constitution. The centerpiece was the idea of "Freedom National". The Constitution, Republicans argued, only recognized and protected slavery in the states, that is, slavery as a condition, as an institution, was only enacted and sustained by local state law. Under the Federal government, slavery was not recognized. Proof of this was how the word was nowhere in the constitution, instead the euphemistic "persons held to service or labor" being used. Consequently, everywhere that was under the purview of the Federal government was free - the high seas, between state lines, the national capital, and most importantly, the territories. If slavery was prohibited in the territories, if they were "free soil", then slavery, with no room to expand, would wither and die. A Republican president could also appoint anti-slavery officeholders to the Southern states, and take other active steps to continuously attack slavery within the Constitution's limits. Soon enough, without the protection of the Slave Power, with no room for expansion, and with the hitherto deluded masses now awake to the evils of slavery, movements to abolish the institution would spring up in every state and eventually result in the destruction of slavery across the United States.

Then, Republicans said that they would not attack slavery directly, but they also candidly said that they would attack it indirectly, that their goal was the ultimate extinction of slavery, and that these methods worked. Some, upon looking at them, may be disappointed, because they all contemplated gradual abolition decades in the future. Was the Southern reaction not justified then? If they just accepted, slavery could continue for decades more. However, Southerners came to believe that Republican policies would have far more sinister and inmediate effects. They agreed on the essential fact that slavery needed to expand and that an anti-slavery Federal government could do a lot to undermine it. The victory of a Republican would open the gates to the enslaved fleeing en masse or trying to revolt, to anti-slavery mails flooding the South, to people "betraying" the South and joining the Republican Party. They, as all reactionaries do, tended to exaggerate the methods and results of the Republican policies, charging that Lincoln was in favor of inmediate abolition, or of slave revolts, or of interracial marriage, etc. More than anything, pro-slavery Southerners simply didn't feel safe in a Union helmed by an openly anti-slavery Party. Even if slavery was not immediately abolished, they had to secede because such a situation would destroy their economic and political power long before slavery was legally abolished.

Secession was not the inmediate response of all Southerners. Some believed that there was no need; others wished to wait for an "overt act". But by stoking popular passions and racial fears, secessionists were able to convince most Southerners that secession was the only appropriate response to the election of these radical anti-slavery men. It was a constitutional right, justly exercised to protect themselves from a sectional party that sought round-about ways to achieve an unconstitutional end. They could not accept Republican rule, with its assertion of slavery's evil, its free soil that seemed to trample on Southern rights, its commitment to fostering anti-slavery feelings in the South. By seceding, Southerners could perpetually secure slavery, and wouldn't have to share a government with a population so openly hostile to the "peculiar institution" so dear to their hearts, economy, and society. There probably wouldn't be any war, they said, and if there was they would easily whip those Yankees. But if they remained in the Union, eventually their society would be shattered. They couldn't wait for an "overt act" - the very election of Lincoln already was one, showing that the Federal government would be, from then on, probably anti-slavery.

Were these fears justified? In some ways, yes. Republicans fully intended to move against slavery, and though these policies wouldn't abolish slavery for years to come, they would surely destabilize Southern society. Maybe they could have voted down slavery indefinitely at the Federal level, but this didn't matter if at the state level slavery was withering thanks to a hostile Federal government. In the view of the secessionists, they had to act. And when they seized the initiative, they pulled the rest of the Lower South along, and then the Upper South after Fort Sumter, when Lincoln had committed the "overt act" by showing his willingness to suppress the rebellion by force of arms instead of surrendering Republican principles. Yet, by doing this they started a war that resulted in a radicalization of Northerners, opening the possibility of slavery being abolished earlier and more radically than it could have been otherwise. In trying to prevent this by a counterrevolution, Southerners sparked the much more radical revolution they were hoping to avoid.