John Brown's raid on Harpers Ferry in 1859 is widely credited by historians with having precipitated both Abraham Lincoln's election in 1860, as President of the U.S., and the Civil War which immediately followed on this election. Although pathetically unsuccessful as a means of causing direct insurrection against slavery in the U.S., the raid, Brown's trial and its aftermath, and the response of both Southern White supremacists and Northern intellectuals such as Emerson and Thoreau, had such a polarizing effect on public opinion that a bloody conflict between North and South became inevitable.
But, let's just suppose that John Brown had decided to stage the raid in 1857, instead of in 1859. Say he manages to speed up his fundraising, and gets sufficient money for the raid in just a year, instead of the two or three years required, OTL. What happens?
I think things might have been rather different. After all, states rights advocate James Buchanan is just one year into his Presidential mandate, and will have three more years in which to try to negotiate a civil solution to the conflicts between North and South. There will be a congressional election in 1858, and the results of this election will no doubt reflect the polarizing effects of John Brown's raid, which likely would have been virtually identical in 1857, to what they were in 1859. So we will have a violently divided Congress in 1859, even more so than OTL. However, we will still have an executive branch seeking conciliation and states rights. And, Congress must seek compromise in order to legislate.
One possibility, is that the South may simply secede at this time, in 1858 or 1859, and Buchanan, as a proponent of states rights, may simply accept secession, without any recourse to using military force, at all, to suppress it. Another possibility is that in an effort to avoid secession and/or civil war, both North and South agree to some kind of compromise position -- say a gradual phasing out of slavery over a period of decades, starting, perhaps, with an automatic manumission of all slaves who have been in service for twenty years or more. In either case, slavery would be likely to continue much longer than it did OTL -- probably until 1900, or even later, in parts of the U.S.
In this context, Brown's timing of the raid in late 1859 may have had a genuinely prophetic and politically brilliant element to it for which John Brown is not generally given credit. John Brown may have chosen, quite knowingly, the perfect moment in which to stage his raid, a raid he knew could not possibly succeed, but, which would inevitably precipitate a catastrophe which would end slavery much earlier than would otherwise have been likely, or possible.