Why is the DC-Baltimore area so lopsidedly blue?

[Sol]

DC and Baltimore are pretty different. DC is pretty Democratic for reasons which everyone has already discussed. Baltimore is not actually that Democratic. Obviously the city is pretty D, as are some of the Southwestern suburbs (which are as much in the DC sphere of influence as Baltimore) but the suburbs which aren't also DC suburbs vote pretty competitively, including the pretty GOP exurban counties of Carroll and Harford.

[Sol]

DC and Baltimore are pretty different. DC is pretty Democratic for reasons which everyone has already discussed. Baltimore is not actually that Democratic. Obviously the city is pretty D, as are some of the Southwestern suburbs (which are as much in the DC sphere of influence as Baltimore) but the suburbs which aren't also DC suburbs vote pretty competitively, including the pretty GOP exurban counties of Carroll and Harford.

Yeah people forget that Harris is the Eastern Shore district but actually came from a suburban district,. His hometown is literally like 15 minutes from Baltimore!  Meanwhile the moderate GOP incumbent that got primaried was from Kent County in the Eastern shore
Del Tachi when he made that poll asking which state was more Southern was partially right in that Maryland is still very much a Southern state outside the DC counties albiet those counties are much too large a portion of Maryland to call it a Southern state.

Agreed, though I'd argue that the DC suburbs are southern in many respects as well--they're obvious sui generis on account of being the nation's capital, but in a lot of ways they operate like a denser, larger version of Atlanta or Raleigh.

[Sol]

Obviously there are aspects of a BosWash corridor culture but DC is very much ''New South'' in my opinion.  Grew rapidly after WWII, lots of new suburbia, and the like which is very UN-Northeastern.  No Italian or ethnic Catholic presence.  PG County doesn't seem very ''Northeastern'' (far more like suburban Atlanta than SE Queens).  It's at least as much like Atlanta as Philadelphia.

If Atlanta was the national capital it wouldn't be all that ''culturally Southern'' either.

It's more than 25% African American, has virtually no white working class and is filled with highly educated people who work for the federal government.  Being (sort of) "in the Northeast" has little to do with the liberalism of the area.  

Another factor that aligns the DC metro more closely with the Northeast, in addition to the obvious infrastructure ties to the Acela Corridor, is the absence of white Evangelical culture. It’s receding even on the Virginia side, but is almost completely absent on the Maryland side until you get out to Anne Arundel County. Maryland’s white Evangelical population is so proportionately low that the percentage resembles New England states more closely than any of the surrounding states.

You can’t really call a place Southern if Evangelical whites struggle to crack double digits in terms of percentage of the population.

I think a lot of that can be explained by the Federal Government thing (a national capitol in Atlanta, as King of Kensington pointed out above, would probably look a lot like DC demographically) and by the fact that much of Maryland is historically Catholic (though I'm not certain precisely the extent of this in many of the DC suburbs so it probably shouldn't be overstated). Southern Louisiana, as I'm sure you know, is predominantly Catholic and is of course Southern despite its distinctive origins.

New Orleans and Baltimore are actually pretty good comparisons as they were both the early large industrial Southern cities which got a lot of immigration.