Voters are deciding Saturday whether Houston will become the largest U.S. city to elect an openly gay mayor.
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.