The single largest failure of our founding fathers was that they did not end slavery. And the argument has always been that what they did accomplish was pushing things to the edge of impossible and taking on slavery too would have been a bridge too far. That always sounded logical to me, and to most historians writing about it too.
But I just read American Creation (Christmas present from one of my daughters) and he talks about this same issue at length - which got me thinking. The founding fathers could not envision a bi-racial society. This was not just in the South (where it was critical) but also it was not envisioned in the North. All of the anti-slavery efforts at that time were to send the slaves back to Africa and it did culminate it the founding of Liberia later on.
What no historian I have read has thought through though is what this logically meant. If the only conceivable way to end slavery was to return slaves to Africa, it made ending slavery impossible. Not expensive, but impossible. First off, the cost just went up substantially because transporting everyone to Africa and giving them enough support to survive would more than double the cost.
But once everyone was gone, what happens to the South? There was no way to replace all that labor and without that labor, the South could not survive. We're not talking a serious recession and contraction - we're talking that the economy would disintegrate. There was no amount of money that could solve this problem.
Issues that could be resolved within the mindset of slavery or emigration were resolved at our founding. The slave trade was set to end 20 years after the ratification of the constitution and there was no significant argument over that clause. Slavery was not allowed in the Louisinia Purchase - by Thomas Jefferson, a Souther slaveholder. And again, no major argument.
Most historians say the Civil War was pushed off as long as possible and only occurred when the South no longer saw "any alternative." But what if instead the Civil War could not happen until enough Americans could accept a bi-racial society? So it was not the South at last deciding to secede but instead enough people finally accepting a bi-racial society. This empowered Northerners to push the South to end slavery and Southerners to see it could credibly be ended. Not immediately, but to clearly set it on the path to extension.
When the Civil War started it was specifically not about ending slavery or civil rights for most in the North (and the South too obviously). But several years into the war when Lincoln declared the Emancipation Proclamation, there was support across the North and among some in the South (every Southern state except South Carolina had regiments fighting on the side of the North too).
Most people probably did not realize it consciously. But subconsciously they had made the transition, that the country could be a bi-racial society. Most clearly did not envision full social equality but they clearly did find full civil equality ok. Look at how quickly Northern states gave African-American (men) the vote.
I don't think there is any way to ever prove or disprove something like this. And almost certainly many factors influenced this in many ways, differently for each person. But I do think it is an interesting question and it could have had a major impact on what happened when.