Ideed. That's the question. Any world without a Civil War is going to be very, very different from OTL. A world where A) the US kept lurching dysfunctionally from crisis to crisis but never had a civil war is very different from B) one where the ARW never happened, slavery was abolished in 1847 with the rest of the Empire, and different again from C) 'go thou my wayward son', and the South is allowed to leave without a fight.
Even if Stephen A. Douglas won the 1860 election, the Civil War was pretty much inevitable at that point - it was only a matter of when it would break out. You'll have to turn the clock back quite a bit more.
Free Power USA and Slave Power USA, One must die for the other to Live. The amazing thing about OTL USA was how long the elective malcontents could keep the rickety structure from collapsing as long as they managed to.