Check out this link from a recent National Journal analysis of the competitive House races. Unlike the in the Compost, there are also Democratic-held districts mentioned that are teetering on the brink of a switch in party control as well. Anyway, this is written by Chuck Todd a respected political analyst. It should answer your questions about which districts are really going to be difficult for the GOP. Northrup's is not one of them by the way nor are several others the Compost Heap mentions.
http://nationaljournal.com/racerankings/house/
Thanks. Just what I'm looking for.
BTW, I see two potential GOP pickups in the top 25 seats they cover.