We've had this debate many times. Historically MD was southern, and many residents still use that historical reference. If you look at measures such as membership rates in non-black evangelical churches or listeners to country music stations, MD doesn't look very southern anymore.
I think those that claim that Maryland isn't "purely" southern have a point, but few make a good case for it being Northeastern.
It is 29% African American. No non-Southern state is more than 20%.
It doesn't have the "white ethnic" populations of the Northeast. Where are the Italians and Irish? How much of the Hispanic population is Puerto Rican?
It also has a lot of unincorporated areas rather than towns/boroughs and the suburbs of DC look more Sunbeltish and "boomburb"-ish than than they look like the "quaint suburban towns and old industrial cities" model that prevails in most of the Northeast.
Of course, being in the national capital region it's going to have a lot of transplants and be an outlier culturally and politically.
Demographically, it resembles Virginia more than it does any other state. Maryland is a border Southern state.
