CONCERN over crime has made the race to be mayor of the nation's fourth-largest metropolis the tightest in a decade.The issue has cut sharply into the job and popularity ratings of Kathryn Whitmire, whose five straight terms in office are the most of any big-city mayor. "She is incredibly durable, and every election has made her more politically powerful," says Robert Stein, a political scientist at Rice University. However, he adds, "this is the first time that job performance has hinged on something that the voters not only care about, but also believe is a responsibility of the mayor." Almost half of Houstonians in a poll would get out of the city if they could, and most of those said crime was the reason. The murder rate rose 24 percent last year, the largest percentage increase of any city. There's one murder every 16 hours on average, although a recent weekend alone had 16 murders. Drugs play a role in 80 percent of Houston crime, Mrs. Whitmire estimates. Houston has replaced Miami as the No. 1 drug port. The city has been designated one of five "high intensity drug trafficking areas" in the country. Contributing to the problem are criminals who are released from the state's overcrowded prisons after serving as little as one-twelfth of their sentences. A disproportionate number are sent to Houston. Over 60 percent commit another crime. Meanwhile, 68 percent of Houstonians say they don't trust elected city officials. A petition with 26,000 signatures forced the City Council to put a term-limitation proposition on the ballot. Although the council responded by adding four competing propositions on term limitations that are more favorable to incumbents, the original is widely expected to pass. If so, a Whitmire victory on Nov. 5 would be her last for the post of mayor. Whitmire has two major opponents in the nonpartisan race for the two-year office. Robert Lanier is a businessman who during the 1980s brought his widely respected acumen to bear on the city's transportation problems, first as chairman of the Texas Highway and Public Transport Commission and later as Whitmire's chairman of the city's Metropolitan Transit Authority. Whitmire issued a proclamation declaring June 22, 1989, to be Robert Lanier day, calling him "the model of an active citizen" and citing "his outstanding and exemplary participation in public affairs." But she fired him as Metro's head less than six months later for trying to put the brakes on plans to build the rail system that she views as crucial to the future growth of downtown and satellite business districts. Lanier believes rail is a billion- dollar boondoggle. Although he helped win voter approval of a plan calling for Metro to develop a rail system, he says research led him to conclude that no one would ride it. Lanier would like to turn Metro into a "general mobility agency," spending even more than the allotted 25 percent of its sales-tax income on street construction and repair. That would free up other city money to hire 600 new police officers within 90 days, which Lanier is sure would dampen crime. That is the number of officers lost since 1987, which Lanier blames on Whitmire for not keeping salaries competitive. Whitmire, while taking steps of her own to increase the number of police, has charged that Lanier favors building roads in the suburbs to enhance the value of numerous properties in which he holds interests. Lanier vehemently denies it, and countercharges that Whitmire is being spurred to support the rail project by contractors and architects who would be paid to create it. The other major challenger for Whitmire's job is Sylvester Turner. A state representative from a 60-percent white northwest Houston district, Mr. Turner is regarded as the first viable black candidate ever to run for mayor here. Born into poverty, Mr. Turner is respected for his self-made achievements: magna cum laude graduate from the University of Houston, and a graduate of Harvard law school. Turner's presence in the race undermines support for Whitmire from a key constituency. The electorate in Houston is divided among conservative whites, roughly 32 percent; liberal whites and blacks, 28 percent each; and Latinos, 12 percent, says Professor Stein. In 1981 and 1985, her two closest contests, Whitmire did not win a majority of the white vote but relied instead on blacks and liberal whites. Whitmire usually captures 75 to 95 percent of the black vote. Turner has been winning endorsements in the black community that had gone to Whitmire in the past. Turner is running a strong third in polls. He opposes the monorail that would be phase one of Metro's rail project, but would consider other rail proposals. He would also work to add 600 officers to the police force. And he'd like to bring some economic development projects to downtown, like a new federal building, a world trade center, and a hotel to serve the massive new but somewhat isolated convention center. Turner has gained notice during televised debates for projecting a moderate image that whites can identify with and for demonstrating a grasp of the issues, Stein says, but he lacks the cash to mount an ad campaign. "Turner has nothing to lose by losing. It's how he loses," Stein says. If Turner can win healthy chunks of the black and liberal white votes, he'll be a strong contender in 1993. Such speculation infuriates Turner, Stein says, because it makes him appear less of a serious candidate for this election and could cause people to vote for another candidate. If the election results in a runoff between Whitmire and Lanier, it will be interesting to see where Turner throws his votes, Stein says. Lanier believes he could get half of Turner's black votes the second time around. But if term limitations were to make Whitmire a lame duck, Turner might prefer to have her win so as not to face an incumbent in 1993, Stein says.