The Richardson City Council directed the City Manager to draft a
City Charter amendment that would require a council member who
announces their candidacy for another office to immediately resign
from the City Council. The amendment, if passed by voters in May,
would apply retroactively, meaning that Council Member Dan Barrios,
who is running for Congress, would automatically be removed from
office.
Can they do this? Setting aside the retroactive nature of the
amendment, they can. ("A home-rule city with two-year terms may
provide in its charter that a mayor or council member who becomes a
candidate for another office automatically vacates the current
office." — Attorney General Opinion
No. GA-02 17.)