Nothing can cut into your credibility faster than flaming Members of Congress for something they aren't even considering doing. Thousands of bills are submitted every year and only a small fraction of them are serious or have a chance of passing. It is a common assumption that letter writing is a meaningless exercise, effort wasted preaching to the birds, and deaf ones at that. In fact, well-timed, informed, and clearly written letters can have an affect on political decisions. On issues that are not on the major agenda, either because it is early in the debate or because the issue seems to be less partisan, members and their staff may be interested in expert opinion.
Most congressional offices keep tallies of constituent positions on issues for exactly that reason.
The common wisdom is that Washington is out of touch with its constituents. The reality is that most successful members of Congress are very closely attuned to public opinion in their districts, in large part because of the daily flood of communication they receive from back home. The impact of a letter is, of course, cumulative. Rarely are there instant conversions. But, there are fewer writers than voters, but a person who takes the time to write is a likely voter.