Have you considered James Buchanan?

He supported the Dred Scott decision and joined with Southern leaders in attempting to admit Kansas to the Union as a slave state under the Lecompton Constitution. In the process, he angered Republicans and alienated many Northern Democrats, bringing the Civil War closer. After Lincoln's election and Southern secession, he failed to defend Union military possessions as it would "anger the South" (as if they weren't already getting on a war footing) and allowed tons of Union ordnance to be outright stolen and delivered into Southern hands. It's hard to think of a more egregious violation of the oath to defend against all enemies, foreign and domestic, whichever you conceive the Confederacy to have been.

Buchanan's sympathies for the South led him to exacerbate the sectional conflicts when instead he could have taken a leadership role in trying to resolve them.

Trump, at least thus far, hasn't brought on a civil war. But the day is young.