There's the bad economy. There's the fact that Republicans are in safe, gerrymandered districts, and mostly have to worry about Tea Party challengers in primaries.
But if you talk to DC insiders in both parties, one idea that regularly comes up is: Bring back earmarks.
Earmarks are basically special provisions in laws that provide some gift or reward to a specific Representative, for their district.
The infamous "Bridge To Nowhere" was a well-known example of earmark abuse, but the thing about earmarks is that it allows sides to "deal." A congressman's vote can be "bought" so to speak. Without earmarks, the only thing a Congressman has is his ideology.