They can only "deliver" if they have the numbers to do it. They can only "deliver" what their constituents say they want delivered. That is why 9 out of 10 people like their congressman or their Senator when it comes to their districts or state. It is when congress acts as a body on national issues that compromise and moderation become necessary.
Seems to me that having someone promise me 50% of what I want and giving me 25% is a hell of a lot better than someone promising me 90% and deliverying 0%.