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

Lincoln would
never have allowed it.  How could he? It was a border state that surrounded Washington DC on
three sides, with rebel Virginia across the river on the 4th side.  If Maryland left, the
capital would be surrounded by a foreign country and effectively under siege.

Lincoln recognized the danger.  When the Baltimore City Council undertook a resolution
supporting secession, Lincoln had them all arrested, including the Mayor, and locked up for two
years without charge or trial.  He did the same with more than 2000 others in Maryland who were
sympathetic to the southern cause, openly breaking the Constitution and reinforcing southern
claims that he was a dictator.  He had no choice, as if Maryland went, the war was likely lost
for the Union.