Washington, D.C.
It would have been an isolated state, surrounded by the Confederacy. It could not have been kept supplied by Washington.
Lincoln didn't want Maryland to secede because Washington would be surrounded by the Confederacy.
The Capital of the US- Washington DC, is SOUTH of Maryland. If Maryland became part of the Confederacy, the US Capital would be surrounded by the Confederate States. And THAT would be rather awkward.
The people of Maryland had a long tradition of slave-holding and ties to the south. They wanted to secede from the Union, but President Lincoln would not allow it. Maryland surrounded the capital of the US on three sides and if they seceded, the capital would have to secede also.
The Border States were the slave-states of the Upper South which narrowly voted against joining the Confederacy. They were Missouri, Kentucky, Maryland and Delaware. Maryland was especially important, because if it had seceded, Washington D.C. would have been totally surrounded by enemy states.To prevent this, Lincon had to break the law by jailing some of their pro-Southern leaders without trial.
The advantages that the Confederacy would have gained if the Border states had seceded would have changed the US Civil War immensely. Excluding Delaware, the slave Border states of Missouri, Kentucky and Maryland would have added 45% to the white population and military manpower the South. The South's industrial base would have increased by 80%. And, very importantly, Washington DC would have been surrounded by two Confederate states, forcing the abandonment of the Union capitol. Also the use of the Tennessee and Cumberland rivers by the Union would have vanished.
Maryland
The verb secede (split from, disaffiliate) forms adjectives with its present or past participle. Depending on the usage, it would be "seceding" or "seceded" (e.g. seceded states that formed the Confederacy).
Lincoln jailed the pro-Southern leaders of Maryland without trial, because they looked as though they would take Maryland into the Confederacy. If that had happened, Washington DC would have been entirely surrounded by enemy states.
It might have to be abandoned, since it would have been totally surrounded by enemy states. That is why Lincoln arrested Maryland politicians who were trying to swing the vote to the Confederacy - actualy breaking Habeas Corpus.
Maryland did not join the Confederacy. However. as a border slave state (like Kentucky, Missouri, and Kansas) Maryland had individuals who fought for the Confederacy during the US Civil War.