Harford is a very Republican county in a very Democratic state. As in Frederick, the population growth that warrants giving additional representation to the area would seemingly be good news for the GOP. Except that it all depends on how the lines are drawn.
The 2010 Census revealed that Harford now has enough population for almost complete two legislative districts--an increase of virtually a full delegate over ten years ago. The above map shows one sample plan for two districts in Harford. Bear in mind that the population figures may be a bit off as Maryland requires that the numbers be readjusted to reallocate the prison population to their last known address.
As shown here, the northern district would be very Republican--it went for McCain over Obama by a margin of 67-31. It somewhat resembles current District 35 except that the current district takes in Bel Air and the map above would place it in the southern district. District 35 is divided into two subdistricts--including one centered on Bel Air which elects one delegate. The senator and all three delegates are Republicans.
The southern district would lean marginally to the Democrats in presidential elections. Obama carried it over McCain by 50-48 but Democrats fared considerably better in state elections. This district resembles current House of Delegates Subdistrict 34A (though that district does not include Bel Air) which now elects one Democrat and one Republican. District 34 has a Republican senator.
One might expect that the new version of District 34 would be highly competitive. Except that I suspect that someone with access to software that allowed one to fine tune the lines to a higher degree than I used here--and no doubt anyone doing this for real would have that access--could draw the map so that it leans more strongly to the Democrats.
Another alternative would be to create a two-member subdistrict that would likely send two Democrats to the House in Annapolis and a one-member subdistrict centered on Bel Air that would be much more likely to elect a Republican even as the Senate seat remained marginal.
In short, the Democrats may well be no worse off--and may even benefit--from gains in representation in a county that leans to the Republicans. The geographic political divide--Republicans tend to live in northern Harford while Democrats are concentrated in southern Harford--facilitates a maps with this result.