Why didn't Maryland join the Confederacy, and what would have happened if it did?

As I understand it, Maryland was very much up for grabs in terms of whether it was going to join the Union or the Confederacy.  There was considerable sentiment for joining the Confederacy.  In fact, a regiment of Union soldiers on its way through Baltimore was actually attacked by a mob, with four soldiers being killed.

In the end, the state stayed with the Union largely because of Lincoln's crackdown on Confederate sympathizers.  He suspended the writ of habeas corpus and jailed many pro-Confederate leaders.

If Maryland had gone the other way, all Northern access to Washington, D.C. would have been cut.  This was the main strategic value of the state  Of course, the added population and economic power of the state would have helped the Confederacy.  Finally, losing Maryland would surely have been a propaganda/morale loss for the Union.