Dear Mr./Ms. Chairperson,
I've formally announced my intent to step down from my position as (politician) to seek the office of (political office).
I will remain until (date), giving my colleagues ample opportunity to (oversee special election/appoint replacement).
As much as I have enjoyed my tenure serving constituents as (position), I've determined I can better effect change as (position). I hope I can count on your support, and I regret that I'm unable to complete my term in office.