It makes sense that you are ultimately the author of this letter. When examining the issue of passionate intensity and forceful voice, no one should supply this. It should come from you because no one will ever be able to articulate your voice like you can because it's yours. Insight and potential direction can be given, but you will have to take this and fuse it into your voice as you construct your letter.
When writing a letter to a legislator, the issue has to be voiced in a manner that it demonstrates a compelling public interest. It should not be a letter that is rooted in an issue that only impacts a few. Rather, the letter and the issue should be something that enables the legislator to see that your voice is representative of the need for the public to understand. Being able to frame personal concerns in the scope of the larger good and the interests of the common good is essential to the advancement of any great liberal democracy. It also becomes quite compelling to the legislator's interest. (One can only hope on this point.)
A way to do this is to open the letter with a rhetorical strategy designed to gain the legislator's interest. A path to pursue would be to offer a personalized view of the interest in question. For example, an issue that I am rather intense about is the plight of homeless children in our public schools. There is something really sad about kids who go to public schools, masquerading the entire day as if they are "like everyone else," but live a life that makes academic success so difficult.
A letter about this topic might open with a narrative of a child who goes through an entire day at school appearing to be like everyone else, only to go "home" to a shelter and is told that they have to pack their meager amount of belongings so that they can move to another shelter or else they will spend the night on the street. All the while, the child is thinking about when they will complete their homework assignment so that they won't receive a detention or be publicly reprimanded by their teacher.
Such an opening, written with precision and delicacy, will be designed to open the eyes of the legislator. It strikes the reader with authenticity and passion. I framed in the interests of public discourse, this will be able to convey your own intensity about an issue.
The next step would be to offer some type of data that suggests how the problem is not isolated. This helps to make it a public issue that the legislator and his colleagues must take seriously. I think that this is important to the legislator as it helps to bring credence to your opening narrative. It also reflects your passion about the topic. If you were passionate about one narrative, it only magnifies when it becomes clear that there are more similar narratives out there.
You might offer potential solutions that you would like to see the specific legislator pursue in chambers and in concert with his colleagues. It would help your cause if you could link something in the legislator's civic or personal history with the topic area. For example, if the legislator has passed similar legislation to address the issue in question, it makes sense that they might be more persuaded to action. It might not be effective to write a letter about homeless children to a legislator who believes that homelessness is not a problem or believes that homeless people are to blame for their own problems. Aligning your letter with the background of the legislator is another way of tailor- making your beliefs towards your audience.
These are some potential steps that can be taken to deliver a rather compelling letter to a legislator about a potential issue that is meaningful to you.