Slavery was peacefully ended in every Eastern state north of the Mason-Dixon Line, beginning at the time of the Revolution and going on until the early 19th Century. It was peacefully excluded from the Northwest Territory from the start. It was excluded from huge areas by the Missouri Compromise, albeit not without significant political controversy. There were numerous attempts to end slavery in various Southern states (including Virginia) well into the 19th Century; slavery seemed to be on its way out in the Border States–it was pretty much a vestige in Delaware, about half gone in Maryland (as many free blacks as slaves by 1860), and seemed to be on its way to ending naturally in Kentucky and especially Missouri. All through the first half of the 19th Century, it was very common for Southerners to profess that they wanted and hoped for slavery to go away, and would get around to getting rid of the institution in their own due time–if only those meddling Yankees would just leave them be. In many cases, these schemes and proposals and vague plans for emancipation seem very half-hearted–as in many of the Northern states, there were proposals for gradual emancipation, with slaves born after a certain date to be freed (and even them, perhaps after some period of servitude), and it was a very common assumption that freed slaves would of course have to be sent away to Africa or some other place far, far away from White America. Schemes for gradual emancipation also left plenty of time for slave owners to sell slaves down South before emanicpation day arrived, and in fact it was very common for more northernly Southerners to openly hope that their slaves (in Kentucky or Maryland or even Virginia) could be peacefully “drained” away to places like Mississippi and Texas, and thus relieve them of the burden of slavery.
But attitudes in the South had greatly hardened by the time secession and war rolled around. It’s a mistake to see history as some simple, unidirectional movement in any direction; just as not everyone in the “olden days” was a Puritan/Victorian about sex until the 60’s, so too there is not a simple progression from wicked unenlightened slave-owners to modern racially egalitarian democrats. Many 18th Century slave owners were far more conflicted on slavery than were the leaders of 1860, however ineffective and hand-wringing and self-serving those 18th Century Thomas Jeffersons may seem to us now–and those attitudes did persist deep into the 19th Century.
But by the time 1860 rolled around, the pro-slavery side had gotten much more radicalized–secession declarations and speeches were full of paeans to the glories of slavery, with no more mention of how slavery was a terrible and regrettable evil, to be gotten rid of, eventually, someday, if only those meddlesome outside agitators would just leave us the heck alone. In some cases, the very people waxing poetic about the benefits of slavery, now and forever, as a permanent and good institution, worthy of pride, not shame, had themselves expressed the view, ten or twenty years before, of slavery as a regrettable, terrible evil, which they of course wished they could somehow be shut of. (And of course, there had always been some unabashed supporters of slavery as a “Positive Good”; conversely, in 1860 there were still those who kept to the “Alas, slavery is a terrible thing, but we cannot get rid of it yet” view–Robert E. Lee is always quoted here, of course.)
As noted, Abraham Lincoln wasn’t a very radical anti-slavery man in 1860; but one sectional clash after another between slave owners and Northerners increasingly sick and tired of being bossed around by slave owners (whatever they may have felt about the slaves themselves) had hardened attitudes. I’m sure John Brown’s raid contributed its bit, but by 1860 there was pretty much no prospect of the South giving up its slaves, nor of the North simply going along with Bleeding Kansas and the Fugitive Slave Laws and the Dred Scott Decision and all the other impositions of the “Slave Power” on free white Americans (along with some genuine and growing movement for abolition on moral grounds). Slavery could maybe have been ended peacefully if they had started back in the days of the Revolution, but it was far too late to escape war by 1860.
Actually, I don’t believe that was true outside of maybe Virginia or South Carolina–in fact, I believe even South Carolina had universal white male suffrage (though there were property qualifications for holding office; and governors and other state officials were elected by the legislatures rather than by the voters directly). In the more aristocratic areas of the South, there were also malapportioned legislatures, with schemes to apportion represenative in state assemblies based on both population and tax revenues (i.e., the amount of property in a given county, i.e., land and slaves); and where older eastern areas (as in Virginia) continued to have the same proportion of delegates as they’d been awarded based on censuses from decades before, with deliberate refusal to reallocate based on any new census, to the detriment of less slave-intensive western areas. On the other hand, in newere parts of the South, it was more common to see a more egalitarian-for-white-males form of racial supremacy championed rather than the class-and-race supremacy found especially in South Carolina: If you were a white man, by gum you were better off than any black man, no matter how humble you were, even if he were some fancy household domestic. (And you therefore had a common interest with the wealthy slaveowner in the big fancy house; besides, those frontier areas, like frontier areas up North, were full of get-rich-quick or at least get-rich-someday optimism, and many people with no or few slaves might hope that someday they would be living in the big fancy plantation house. And of course poor whites has no wish to compete with millions of freed blacks, economically, socially, and politically.)