NEW ORLEANS— In the St. Thomas housing project, with its rows of boarded-up red brick buildings, the ghosts speak more eloquently than the living about the despair at the bottom of urban America.

There's the spindly crape myrtle tree and a modest plaque marking the spot where 5-year-old Coco Vessel died in a crossfire of bullets on Nov. 19, 1989. There's a weed-choked patch outside the apartment where Elnora Barnes for many years tended her begonias and roses, green peppers, eggplant and okra before gunfire drove her away.

There's no missing the dreadful toll of inner-city decay in America's urban slums. But for those outside the blighted districts, insulated by money and geography, there's a terrible paradox. As the problems of the urban poor become more toxic to those inside, their immediate impact on most Americans is becoming more distant and elusive. Effects Are Widely Felt

In a nation now more suburban than urban, with residential segregation common, jobs increasingly following residences out from the urban core, and regional growth and corresponding political power moving away from the most affected areas, the impact of urban poverty is more like a tornado than a hurricane, ripping apart some cities, neighborhoods and lives, but leaving others almost intact. Far-Reaching Effects

But even though elusive, the effects of urban poverty can be far-reaching. They strain big-city budgets, produce criminal behavior that is altering urban life, poison race relations and threaten to create an undereducated work force that could haunt the nation for decades. The presence of the homeless, like battered refugees from a blighted kingdom, in even the most prosperous cities serves as a reminder of the depth of the nation's social ills.

One of the political challenges of the 1990's is finding a way to summon the national will to address a problem that the nation has done its best to isolate and flee.

"After the riots of the 1960's, a quite elaborate social reorganization of American metropolitan life has taken place based on the principle of everbody getting as far away from the underclass as possible," said Nicholas Lemann, author "The Promised Land," a book on poverty to be published in March by Alfred A. Knopf.

"A place like New York is a very special case where the richest neighborhood is right next to some of the poorest," Mr. Lemann said. "That's very rare. Most people know the conditions in the ghetto are terrible, but it's a problem that doesn't directly affect them." View From the Projects A Daily Assault Of Fear and Despair

To Lilianne Flavin, a Dominican sister from Ireland who is an incongruous, healing presence at the St. Thomas housing project here, the urgency of the problem is as unmistakable as the despair she sees daily. "The question shouldn't be what these people are doing to society," she says. "It's what society is doing to them."

St. Thomas was once a stable home to white working class residents, with neat three-story brick buildings, French Quarter-style iron grillwork and leafy oaks. Now, about 550 of the 1,500 apartments are gutted and abandoned and the project is a patchwork of decay -- an uprooted toilet in front of one building, a pile of dilapidated furniture forming a makeshift playground in front of another. Perils to the Children

The overwhelming majority of the residents are the victims, not the perpetrators. They are people like Rose Bailey, worried about the lead poisoning of her 3-year-old son, Theresa Delmore, hoping her four children can escape the crack to which she is addicted, or Earnestine Lather, resolutely trying to protect her children from the slum's perils.

"Being here, I feel like we're living in the Last Days," said Mrs. Lather, invoking a term from Revelation to describe the project's slide toward chaos.

As she waited to meet her 6-year-old's school bus, Mrs. Lather added, "Mostly, you try to keep them away from the drugs and the violence, but it's hard. I tell my oldest boy I don't want him hanging out with the boys who are getting in trouble, and he says, 'Aw, mama, ain't nobody else for me to be with.' "

There is rage at St. Thomas. Its explodes in the gunfire at night and lurks just beneath the surface of daily life, evident in the touch football game played each afternoon with a fury that's more blood sport than play. 'It's Like Being in Jail'

But far more common is simple despair.

"I feel like I'm a different person when I'm not here," said Ms. Delmore, who stopped working as a nurse's assistant when she became addicted to crack three years ago. "I feel good. I feel I don't need drugs. But being in here, you just feel like you're drowning. It's like being in jail. I hate the projects. I hate this rathole." View From Next Door Cities Transform As Many Escape

New Orleans is one of the rare cities where rich and poor live in close proximity and where conditions at the bottom have a direct effect on everyone. Fear of crime is transforming some neighborhoods, like the lovely Uptown district near Tulane University, from some of the nation's most appealing to some of its most besieged.