James Buchanan - allowed Democrat sore losers to engage in widespread, organized insurrection and rebellion, ultimately resulting in all out civil war... there's also evidence that he was aware the state department had been disproportionately reinforcing southern military installations from 1857-1860 under the guise of "fearing slave revolts", when in reality Democrat slavers were prepping for their own rebellion. What makes Buchanan such a shit, is that he sent an army to Utah to get the Mormons in line over bigamy yet as the deep south conspired to plunge the nation into war, he not only let it happen but implicitly condoned it.

Had Buchanan crushed the fireeaters immediately, perhaps the subsequent events are prevented and we never end up with massive shits like Wilson, FDR, LBJ, Willy and Hussein.

Tyler and Polk are up there as well... Tyler literally made himself President and Polk provoked war with Mexico for the sole purpose of gaining territory to open up to the expansion of slavery.