I am a believer that the greatest asset of the Civil War for the North was not their industrial capacity, or their numbers, but was Abe Lincoln himself, I think he was one of the few men who could hold a coalition of Radical Republicans and War Democrats together and masterfully waiting for the proper time to push for abolition, which kept the border states from joining the Confederacy.
So, let's say Baltimore Plot goes off (and I know there is debate whether it actually was a real plot, but let's just go with it), and now we have President Hannibal Hamlin, who from what I've read about him, was not the same as Lincoln. He and Lincoln got along personally, but Hamlin was identified with the more radical clique of Republicans in Congress. His major action during the war was pushing for the appointment of Hooker as commander of the AOP (which of course did not end well). I could see this group pressuring him to let Fremont's emancipation edict stand. In fact similar orders might be replicated in Kentucky and Maryland.
So, if Hamlin decides to let loose the dogs of emancipation in the early stages of the war, could the border states of Missouri and Kentucky (maybe even Maryland?) be scared into the arms of the Confederates (let's also say Polk doesn't make his stupid move against Kentucky in September)?