The giant billboards on I-45 north of downtown do not truck with nuance.

"Save Houston from the Sodomites," implores one. "God destroyed Sodom and Gomorrah for homosexual sin," admonishes another.

Residents of the nation's fourth-largest city go to the polls today to vote on an ordinance that would protect homosexuals from discrimination in municipal employment practices; but, as the billboards testify, Houstonians are being asked to weigh the referendum as though it were about the homosexual life style.

So framed, the public debate here in the last three weeks has been as loud and divisive as any since the civil rights struggles.

Mayor Kathy Whitmire, a supporter of the ordinance, has accused opponents of being filled with "hate and venom" and has broken bitterly with the city's business establishment over the issue.

On the other side, one medical witness has testified to the city council that homosexuals ought to be quarantined, and another witness testified in lurid detail this week about the "diabolical" sex acts that "cruel homosexual abominators" engage in with animals.

In the past 15 years, more than 40 U.S. cities have expanded the antidiscrimination portions of their civil service codes to include sexual preference. With the exception of a 1977 fight led by entertainer Anita Bryant in Florida's Dade County (Miami), most cities have acted without incident.

But now, national gay leaders worry that several factors -- including public fears about acquired immune deficiency syndrome (AIDS), a deadly disease that strikes mainly homosexuals and bisexuals; the growing political activism of religious fundamentalists; and the return of so-called "family values" -- have triggered a backlash that could overwhelm the precarious tolerance that homosexuals now enjoy in some urban areas.

"This has been a real eye-opener to a lot of us," said Sue Lovell, president of Houston's 10-year-old Gay Political Caucus. "Gays here got complacent. We're seeing now that the community is not as tolerant of us as we thought. I know I'm shocked the business community here reacted the way it did."

Despite its macho cowboy-and-oil image, Houston for years has been home to the most visible and politically active gay community in the Southwest. "This is an open town," said George Greanias, a city councilman whose district includes Montrose, the city's gay neighborhood. "We value people for their ability to produce, to make money."

Given that social Darwinian tolerance, gays find business' opposition to the referendum particularly nettlesome. Even Councilman John Goodner, a leader of antihomosexual opinion, concedes that "gays in Houston have higher incomes, higher education and more disposable income than the community at large." But he worries that passage of the ordinance would turn the city into a gay "mecca" and stunt its economic growth.

The Chamber of Commerce agrees. Concerned about the "detrimental effects" that the ordinance might have on Houston's "future public image and economic climate," its executive committee voted last month to oppose the referendum. Business leaders have ponied up most of the estimated $400,000 spent on a television and radio campaign opposing the ordinance.

Other forces also have jumped on the antiordinance bandwagon. Local Republican Party leader Russ Mather has been a strong opponent, as have members of the black clergy, who made both the moral and the we-need-to-worry-about-our-own-slice-of-the-economic-pie arguments against the ordinance.

Then there have been the fringe groups -- the Ku Klux Klan, which led an overflow audience in a chant of "Kill the queers" as the city council passed the ordinance in June, 9 to 6; and the authors of widely circulated leaflets, one of which depicts a little girl about to be beheaded by an ax-wielder, with the caption: "You are 15 times more likely to be murdered by a gay than a heterosexual during a sexual murder spree," and another showing a boy being yanked into a bathrooom stall, with the caption: "Homosexuals don't procreate, they recruit -- our children."

After passing the ordinance last June, the council withdrew it after opponents gathered more than 60,000 signatures to force today's referendum.

Given the ugly turn of the debate, some gay rights activists and many business leaders now say privately that the idea of an ordinance was a bad political miscalculation. "They broke the first rule of politics -- 'If it ain't broke, don't fix it,' " said one corporate executive who asked not to be identified.

"They got friends in city hall; they're a respected force in politics, and they tried to rub people's noses in their power. Now their bill is going to get beat, and it's going to make Houston look like an intolerant place, which no one wants. A lot of us have taken a pox-on-both-your-houses attitude toward the chamber and the gays. We're staying out of it."

The genesis of the ordinance is rich in city hall intrigue. It was introduced by Councilman Anthony Hall, who is said to harbor mayoral ambitions. Hall was elected to an at-large seat last year without GPC support, and Councilman Goodner claims that Hall pushed the ordinance to build a base of gays and blacks to enhance his election chances. Hall denies that. The GPC says only that it never asked for the measure in the first place.

"It's true that a lot of gays might have preferred this never come up," Greanias said. "But now that it has, it's also true that there's a feeling that there is no going back."

Of all the arguments against the measure, the one that most irks Greanias is Goodner's claim that it is unnecessary because there are no known cases of discrimination against homosexuals in city employment.

"That's the classic Catch-22," Greanias said. "The reason there is no documentation of discrimination is there is no grievance procedure." He said homosexual employes have complained to him about discrimination, "but most are afraid to come forward."

Goodner, a soft-spoken businessman who once sought and received the GPC's endorsement, said passage of the ordinance would embolden homosexuals to become a "political juggernaut."

"If they win this time, they'll be back asking for affirmative action, for housing protection, for changes in the curriculum in the schools," he said.

GPC's Lovell says that Goodner overstates the power of the group, which she said has a mailing list of fewer than 15,000 names (in a city of 1.6 million). She also said the GPC has "no long-term agenda. Our thinking was we had a city policy that we wanted to codify into law. When you have a chance to do that, you take it."

"Some people say the ordinance was unneccessary," she added. "But if anyone ever wondered why we need the protection, just look at the backlash."

Lovell added that if the ordinance fails, "It might be open season on gays for a while."