Dear Mr./Ms. Party Chair,
It has been a long and difficult decision, but I feel that it is time that I tender my resignation as (title), effective (date). I understand that my term is set to run until (date), but due to (reason) I feel that it is time for me to step down.
As an elected official, I know that much is still expected of me. I would like to meet with you sometime in the next week and discuss the steps to terminate my position in office and organize arrangements to fill the post. Thank you for your understanding, and I look forward to speaking with you on this matter soon.