I like your letter – you’ve gotten some good suggestions for making it better. In politics there are always winners and losers for any policy decision, and politicians need to hear from the people they’re hurting.
One thing I’d suggest, don’t threaten to stop voting. You’re only a constituent if you’ll vote for them. Tell them you’ll never vote for _them_ if they vote for these bailouts, but if you say you’ll stop voting entirely they don’t have much reason to care about your opinion. The real threat is to promise to work hard to support a challenger in the primary elections.