The North was bit slow to realize what the war meant and how it would have to be handled. Maryland's secession could well backfire badly. For one thing, instead of the First Battle of Bull Run the Army of the Potomoc is deployed to occupy Maryland and put down secession. Given the tight division in Maryland this would be relatively simple, ESPECIALLY since strong forces loyal to the union would be seeing the price rapidly mounting.
1) Maryland's panhandle would be immediately lost, in a fashion similar to West Virginia if Maryland wasn't put down fast.
2) The Army of the Potomoc would occupy the major cities, not to mention reinforce DC. Resistance would be extremely unlikely, especially in a state divided almost evenly.
3) Property values would collapse. In our history, a major anti-secession voice emerged after the riots in Baltimore caused the city to (briefly) lose an estimated one-third of its value.
4) Slavery, already at risk in Maryland, would crumble with most slaves within days/hours of free states. Possibly the potential of this manpower would hasten certain proclamations regarding the end of slavery.
5) With this diversion the Union would likely spend more time on defenses for DC and avoid some of the early embarassments of 1861. Even worse, from the South's perspective, the need for some kind of action might have gotten the Union moving into Tennessee sooner.
Xen, it was the Army of Northern Virginia under Lee, and it didn't exist until several smaller Confederate forces united for the first Battle of Bull Run. The likelihood of these forces suddenly being pulled together to attempt a forcing of the Potomac, in the face of superior Union numbers and the American Navy would be very low. Perhaps an assault on Harper's Ferry?
How would that work out? Quick defeat for the Union or some bloody early debacle for the South instead of a victory?