The runoff election pits City Controller Annise Parker against former city attorney Gene Locke.
Parker is a lesbian who has never made a secret or an issue of her sexual orientation.
But in recent weeks, anti-gay activists and conservative religious groups have endorsed the 61-year-old Locke and sent out mailers condemning Parker's "homosexual behavior."
Meanwhile, gay and lesbian political organizations around the country have rallied to support the 53-year-old Parker.