WASHINGTON, Jan. 26 — New Orleans could lose as much as 80 percent of its black population if its most damaged neighborhoods are not rebuilt and if there is not significant government assistance to help poor people return, a detailed analysis by Brown University has concluded.
Combining data from the 2000 census with federal damage assessment maps, the study provides a new level of specificity about Hurricane Katrina's effect on the city's worst-flooded areas, which were heavily populated by low-income black people.
Of the 354,000 people who lived in New Orleans neighborhoods where the subsequent damage was moderate to severe, 75 percent were black, 29 percent lived below the poverty line, more than 10 percent were unemployed, and more than half were renters, the study found.
The report's author, John R. Logan, concluded that as much as 80 percent of the city's black population might not return for several reasons: their neighborhoods would not be rebuilt, they would be unable to afford the relocation costs, or they would put down roots in other cities.
For similar reasons, as much as half of the city's white population might not return, Dr. Logan concluded.
"The continuing question about the hurricane is this: Whose city will be rebuilt?" Dr. Logan, a professor of sociology, writes in the report.
If the projections are realized, the New Orleans population will shrink to about 140,000 from its prehurricane level of 484,000, and the city, nearly 70 percent black before the storm, will become majority white.
The study, financed by a grant from the National Science Foundation, was released Thursday, 10 days after the mayor of New Orleans, C. Ray Nagin, who is black, told an audience that "this city will be a majority African-American city; it's the way God wants it to be."
Mr. Nagin's remark was widely viewed as an effort to address criticism of a proposal by his own rebuilding panel, the Bring New Orleans Back Commission, that calls for a four-month building moratorium in heavily damaged areas. He said later that he had not meant to suggest that white people would not be encouraged to return.
"Certainly Mayor Nagin's comments reflected a concern on the ground about the future of the city," Dr. Logan said. "My report shows that there is a basis for that concern."
The study coincides with growing uncertainty about what government assistance will be available for property owners and renters. Louisiana will receive $6.2 billion in federal block grants under an aid package approved by Congress in December, part of which will be used to help homeowners. But that will not be enough money to help all property owners in storm-damaged areas, Louisiana officials say.
Those officials have urged Congress to enact legislation proposed by Representative Richard H. Baker, Republican of Louisiana, creating a corporation that would use bond proceeds to reimburse property owners for part of their mortgages, then redevelop the property. But the Bush administration has said it opposes the bill, out of concerns that it would be too expensive and would create a new government bureaucracy.
Asked Thursday about his opposition to the measure, President Bush told reporters that the $85 billion already allocated for Gulf Coast restoration was "a good start." He added that he was concerned that Louisiana did not have a clear recovery plan in place.
But Gov. Kathleen Babineaux Blanco of Louisiana, a Democrat who has clashed frequently with the White House, said Mr. Baker's bill provided a clear plan.
"Administration officials do not understand the suffering of the people of Louisiana," Ms. Blanco said in a statement.
Demographers are divided over the likelihood of a drastic shift in New Orleans's population. William H. Frey, a demographer at the Brookings Institution who has studied the hurricane's impact on the city, called Dr. Logan's projections "a worst-case scenario that will come about only if these evacuees see that they have no voice in what is going on."
But Dr. Frey also said low-income evacuees might indeed begin to put down roots in cities like Houston or Dallas if they did not see movement toward reconstruction in the next six months.
Elliott B. Stonecipher, a political consultant and demographer from Shreveport, La., said that unless New Orleans built housing in flood-protected areas for low-income residents, and also provided support for poor people to relocate, chances were good that many low-income blacks would not return.
"If they didn't have enough resources to get out before the storm," Mr. Stonecipher said, "how can we expect them to have the wherewithal to return?"