Houston voters could elect the city's first openly gay mayor Saturday when they cast ballots in a hotly contested runoff election marked by fierce campaigning and anti-gay rhetoric.

City Controller Annise Parker, who is running against former city attorney Gene Locke, is a lesbian who has never made a secret or an issue of her sexual orientation. If she wins, Houston will become the largest U.S. city ever to have an openly gay mayor.

Parker's sexual orientation became focus of the race in the Texas city in recent weeks after anti-gay activists and conservative religious groups endorsed the 61-year-old Locke and sent out mailers condemning Parker's "homosexual behavior."

Meanwhile, gay and lesbian political organizations nationwide have rallied to support the 53-year-old Parker by raising money for her campaign and making calls urging people to vote.

Locke, who would become the city's second black mayor if elected, has been trying to distance himself from the anti-gay attacks while courting conservative voters who could tip the race in his favor.

Although Locke has condemned the divisive rhetoric, two of his key supporters contributed money to a conservative political action committee that sent out an anti-gay mailer earlier this month, urging voters not to pick Parker because she was endorsed by the "gay and lesbian political caucus."

Campaign finance reports show Ned Holmes, finance chairman of Locke's campaign, and James Dannenbaum, a member of the campaign's finance committee, each gave $20,000.

A poll released by Rice University this week showed Parker leading Locke 49 percent to 36 percent. The poll, commissioned by television station KHOU and radio station KUHF, is based on telephone interviews earlier this week with 442 registered voters in Houston. It has an error rate of plus or minus 4.7 percent.

Parker and Locke, both Democrats in the nonpartisan race, made it to the runoff after garnering more votes than two other candidates on Nov. 3. They are vying to replace Bill White, who is term-limited after serving six years and is seeking the Democratic nomination for governor.

Several smaller U.S. cities have openly gay mayors, including Portland, Oregon, Providence, Rhode Island, and Cambridge, Massachusetts.

Houston, the country's fourth largest city, is predominantly Democratic and about 25 percent black and one-third Hispanic. About 60,000 of its 2.2 million residents identify themselves as gay or lesbian.