Answer:
All citizens of Westport County.
Step-by-step explanation:
Although this letter originated from Northfield, it is meant to address all citizens of Westport County. We know this because Proposition S intends to affect all public services throughout Westport County. Moreover, the author tells us that he is joining his "fellow residents of Northfield in urging all citizens of Westport County to vote No on Proposition S in the upcoming election."