I wonder about constituent letters, based on my own experience. I just about never communicate with my Senators and Representatives, because they’re liberal, and I feel like I’d just be wasting my time. I think they’d only react if contributors were bitching, not constituents at large.
I write my Representative all the time and let him know that he’s earning my vote based on his legislative actions. he’s a good guy that well represents his constituents and I think most of the correspondence that he gets from his constituents reflect that. If that correspondence were to suddenly turn sour over one issue right before an election, I suspect that he would become quite responsive.