Mr Congressman, you have lost my vote forever. Furthermore, I will be campaigning on behalf of $OPPONENT who still believes in freedom and our greatest liberties. The fact that we have great men like $OPPONENT proves American democracy still works.
"Lost my vote forever" is, I'm sure, the kind of meaningless spiel they hear everyday. It would likely have the same effect on a congressman that saying "meat is murder" has on me - instant tune-out.
A response informing the congressman of why he's wrong, citing loss of jobs and liberty while threatening innovation, all in the name of an ever-growing government keen to erode our rights in the service of special interests, would likely hit enough republican talking points to at least merit a moment's consideration. Which, at the end of the day, is probably the best you could hope for.
That's assuming that this comgressman believes his talking points. I'm of the opinion that many talking points are for _other_ people to believe, but the ones saying them do not believe in them. The subject matter just doesn't affect them. Why would a politician who has been bought by Big Media care about innovation, loss of jobs/liberty, or erosion of rights?
Hit them where it hurts. Threaten _their_ job. If you lose your job due to their legislation, they'll just blame their opponent and tell you to stop begging for handouts.
Threatening their job is one thing -- it's when you say that they've lost your vote _forever_ that it becomes too far, as they might feel that they have no choice for recompense to gain your vote back.
In short, it's going too far, and even if it isn't, certainly seems like an idle threat.
Saying something more like "I cannot vote for any candidate who supports this policy or any like it" is perhaps a more effective statement to make.
"Every one admits how praiseworthy it is in a prince to keep faith, and to live with integrity and not with craft. Nevertheless our experience has been that those princes who have done great things have held good faith of little account, and have known how to circumvent the intellect of men by craft, and in the end have overcome those who have relied on their word.... he who has known best how to [be foxy] has succeeded best.
"But it is necessary to know well how to disguise this characteristic, and to be a great pretender and dissembler; and men are so simple, and so subject to present necessities, that he who seeks to deceive will always find someone who will allow himself to be deceived. One recent example I cannot pass over in silence. Alexander VI did nothing else but deceive men, nor ever thought of doing otherwise, and he always found victims; for there never was a man who had greater power in asserting, or who with greater oaths would affirm a thing, yet would observe it less; nevertheless his deceits always succeeded according to his wishes, because he well understood this side of mankind.
"Therefore it is unnecessary for a prince to have all the good qualities I have enumerated, but it is very necessary to appear to have them."
Unfortunately it is incredibly rare to find a match for your hypothetical $OPPONENT that is both electable enough that your letter won't be laughed off and who actually fits the description of being a great man (or woman).
This is often the problem with politics, you may strongly disagree with government policy on an issue that is important to you but all of the other parties that stand any chance at all of being elected to office also support the governments stance on that issue.
So your choice is either to concede that issue or vote for a fringe party (which here in the UK basically makes your vote worthless).
Freedom was not nor ever will be won with words. Freedom is won from the barrell of a gun. words only have power when they are from someone ready to use action if they have no effect.
