Looks like he won the seat in a gerrymandered district. This manipulation of district boundaries takes advantage of people who vote along party lines regardless of an individual candidate's qualifications.
In short, gerrymandering, closed primaries, and private campaign finance form the Triangle of Doom. In combination, they provide the noose with which Congress is choking America to death. Abuse at the hands of the NSA (and their multi-billion dollar web of contractors) is just the latest, and perhaps the most chilling example of this phenomena.
"In 2012 the district was found to be the eleventh least compact congressional district in the United States."
And a link to this article, which shows that district, and is titled "Maryland has least compact congressional districts in nation": http://marylandreporter.com/2012/10/03/maryland-has-least-co...
Now, in all fairness they do have a cragy coast, but it is very clear the districts are unconscionably gamed.
I'm not sure how it was done in the source for that news article, but gerrymandering-detection algorithms should ignore natural borders in that regard. See, for example, this paper which measures gerrymandering in terms of convexity: http://mathdl.maa.org/images/upload_library/22/Polya/Hodge20...
Primary him. Primary him credibly. Primary him principally on this issue.