I've always wondered if the Republican President who did the most harm might be the first one. If Lincoln had just let the South secede we would have Canada with decent weather.
I admit that the confederate states of america would have been a hell hole for a while, but with the underground railroad as long as a subway line, African Confederate Americans would have mostly managed to escape and the remaining confederates would have have reaped the wages of contempt for progress and education (mainly cotton, as in, you reap what we sew).
I'd say pretty soon they would have applied for re-admission and accepted a reasonable deal, such as this: every White person born south of the Mason-Dixon line (such as myself) counts as three fifths of a man.
Instead honest Abe managed to make his degenerate successors almost unbeatable as soon as the neo confederates got over their anger at the emancipation proclamation.