I think Muon has the point here. Trying to divide these states as entirely northern or southern misses the regional differences of each. Yes, there are parts of rural Delaware and Maryland that are closest to southern culture (and to a lesser degree, southern politics). But the same could be said of much of SW PA south of Pittsburgh. And Ohio? A friend of mine once said it could be argued the real divide between north and south isn't the Mason-Dixon Line, but I-70.
If I had to place MD and DE in one region it would be the north based both on it's dominant politics (Baltimore and the DC suburbs are far more indicative of MD culture than the East Shore; Natch Wilmington--essentially an outlet of greater Philly--in DE vs. downstate. Ditto for historical reasons. Both fought for the Union notwithstanding a minority of Confederate sympathizers. Still, ask me about the East Shore or Sussex County and you'll rightly get a different answer--just like say Meigs County Ohio. 
Agreed and will add...
Politics and culture overlap, but are not one in the same. MD and DE shouldn't be considered part of the South, not just because they vote differently, but they don't really classify as culturally southern.
Fla., N.C., and Va. have pockets that are not culturally southern, but the majority of the people living in those states would be classified that way.
