Both sides will not concede if the other candidate wins. Trump supporters will not accept a Biden win...
Trump will not stand for it, then the left will say "He's refusing to accept the election results!" Cue the protests, riots, violence, coup; etc. This will likely incite a civil war.
If Trump wins, we know all hell will break loose with the left. They will accuse him of cheating, as they have from day 1, and again cue the protests, riots, a coup - you name it. Most likely we will face a civil war in either scenario. Unless we somehow secede or separate prior to war breaking out, thus ending the United States as we know it. War is always a horror. Civil war is even worse. We may think it will be an easy win, but there will be a heavy price to pay.