If the election was thrown to the House in 1860, there is a narrow window that Breckenridge could be elected if state delegations controlled by ex-Whigs support him or Lane could be elected by the Senate choosing him as Vice-President and having the clock run out for choosing a President.
I think that barring a miracle, the Democrats are sure to get tossed out in 1864.
So, the question is, what is the maximum amount of damage a pro-slavery Southern Democrat President could do in four years? Any by "damage" I obviously mean via legal and constitutional means (although, questionable actions that could be defended as constitutional might be acceptable).