Use APA style manual. Give specific reasons for supporting or opposing a bill. Be concise and straightforward. One page is sufficient in your letter. Mistakes in spelling and grammar should not be made.
Sponsorship is a commitment.
While you may not be able to speak directly to the legislator, the legislator's Aide is usually very helpful and accommodating. It is your personal position you are writing about, not your employers. Legislators do not want to do anyone's academic work for them. Additionally, Seton Hall University itself may have a different position on the bill. The reverse is also true.