Usage patterns
〈人〉が市議に当選するfor someone to be elected as a city councillor
She was elected as a city councillor in last month's election.
〈人〉が市議を務めるto serve as a city councillor
My father served as a city councillor for ten years.
〈人〉が市議に立候補するto run as a candidate for city councillor
He intends to run for city councillor in the next election.