If ever there was a moment for the obscure federal flood insurance program to ride to the rescue, it would seem to have been in the aftermath of Hurricane Katrina.

Its premiums were supposed to insure homeowners in flood-prone areas and also protect taxpayers from spending billions to bail out flood victims. But with Hurricane Katrina, the program failed on both counts.

Nearly half the victims did not even have flood insurance. Claims from homeowners who were insured, $25 billion worth, bankrupted the program. And the government has had to commit $15 billion in additional taxpayer money for rebuilding in Louisiana and Mississippi.

Now, an effort to rescue the insurance program that grew in Hurricane Katrina's wake is faltering, too. Though experts foresee a generation of fiercer and more frequent storms, Congress seems unlikely to make more than modest changes when it takes up the program in the coming weeks.

The drive to restructure the perennially underfinanced program has been blocked by real estate interests, who worry that requiring millions more people to buy flood insurance would stifle development, and by lawmakers from areas that rarely flood who see their constituents as supporting those who are frequently flooded, particularly in the South.

"You've got people living in dry areas paying for people who want to keep living in wet ones," said Representative Candice S. Miller, Republican of Michigan. "They're sticking it to us, and I don't like to be stuck."

The inability over decades to work out who pays the bill for flooding is at the heart of the weaknesses in the insurance program so blatantly laid bare by Hurricane Katrina.

A close examination of the program shows how those same lobbying pressures and regional rivalries have helped create an insurance plan that has consistently defied the central rule of how to succeed in the insurance trade: have enough policyholders paying enough in premiums to spread out the risk and build a financial cushion against disaster.

Since its beginning in the late 1960's, the flood program has struggled against a basic handicap: Most people, except those in the clearest path of danger, believe they do not need it.

So, in an effort to make the insurance affordable and attractive to reluctant homeowners, the government has kept premiums artificially low typically $300 to $400 a year for coverage up to $250,000. At the same time, though, it has limited the size of the program's flood zones the only areas in which many people are required to buy the policies.

Testifying before Congress this year, the flood program's acting director, David I. Maurstad, acknowledged that it faced "numerous challenges on a variety of fronts." But insurance officials also say the program, which takes in only $2.2 billion in fees each year, was never meant to handle a devastating storm like Hurricane Katrina.

Even so, many of the program's critics in Congress and elsewhere argue that it can and should be rebuilt as a stronger hedge against a less-catastrophic run of storms.

"It hasn't come close to its promise of insuring everyone who's in danger of being flooded, reducing the cost of disasters for the federal government or making sure the program ultimately pays its own way," said J. Robert Hunter, who once ran the program and is director of insurance for the Consumer Federation of America.

The idea of federal flood insurance began in the early 1950's with President Harry S. Truman, after big floods in his home state, Missouri. Private insurers would not provide the coverage, arguing that only those most likely to be flooded would buy it, and that an avalanche of claims would bring big financial losses.

But what finally brought the idea to fruition nearly two decades later was a desire to do something about the rising cost of federal disaster relief. In particular, Congress felt it was important that the beneficiaries helped pay the bill.

Under the program, the Federal Emergency Management Agency maps areas along coasts, lakes and rivers with significant flood risk a 1 percent chance of flooding in any year and tries to sell insurance to people in or near them. Though more than 4.8 million people have the policies, that includes only about half the households in the flood zones.

To attract buyers, the government discounted the premiums, some to a fraction of what a private company would have charged. But initially, no one was required to buy flood insurance, and hardly anyone did.

Gilbert F. White, a retired professor whose research in the mid-1960's laid the foundation for the program, said the government originally considered larger flood zones but pulled back under pressure from homebuilders and real estate developers.

The program also linked the availability of insurance to agreements by local governments to enact new building codes intended to reduce flood damage, mainly by raising houses above expected flood levels. But the real estate interests persuaded many communities to stay out of the program, Mr. Bernstein said.

In the summer of 1972, Hurricane Agnes caused $400 million in flood damage along the Eastern Seaboard; only $5 million was covered by insurance. Soon after, Congress made the insurance mandatory for people in the flood zones with federally regulated mortgages.

These measures let the program expand just enough to limp along. Over the years, it paid out almost $15 billion in claims, borrowing from the Treasury in bad years and paying the money back, with interest, in quieter times. But by early last year, the fund had slipped into the red again, drained by $2 billion in claims from the hurricanes in Florida in 2004.

Along the Mississippi coast, waves up to 30 feet high surged past the flood-zone boundaries. Federal figures show that two-thirds of the 46,000 flooded homes and apartments in Mississippi were outside the zones. One problem was the flood maps, which had not been updated in more than a decade and no longer reflected the true danger.

In New Orleans, the program stumbled on another assumption: though experts had long warned that a terrible hurricane could top the levees and inundate the low-lying city, that possibility was never factored into the hazard calculations. Instead, the mapmakers focused solely on the impact of heavy rains.

Luckily for New Orleans, which often floods in bad rains, the bulk of the city was in the flood zones. So 65 percent of the 54,000 flooded homeowners had at least some insurance.

But a few areas that flooded badly from the levee breaks, like much of the Lower Ninth Ward, were outside the zones. Relatively few people there, or among the city's large population of renters, had insurance. And many others had only enough to cover their mortgages, leaving little cash to rebuild.

Joy Fortune, a retired bank executive, and her 86-year-old mother, Hazel Castanel, had a $73,000 flood policy on their beige brick home. But after the house sat in six feet of water for two weeks, an adjuster estimated the damage at $124,000, leaving them $51,000 short.

At first, it looked as if they would not be able to rebuild. But then the Bush administration and Congress came up with $15 billion in grants to make up for gaps in insurance. "We want to go back," Ms. Fortune said. "It's my home and it's our life."

Even with the gaps, the program was swamped by $25 billion in claims from Hurricane Katrina and two other hurricanes last fall, and it will need to borrow all that money from the government to pay them. What is more, insurance officials say they doubt they will be able to pay much of it back, leaving taxpayers with the bill just as if there were no insurance program.

It was with that huge bill in mind that two influential congressmen Michael G. Oxley, an Ohio Republican who heads the Financial Services Committee, and Barney Frank, a Massachusetts Democrat drafted legislation that would have greatly expanded the flood zones to draw millions more homeowners into the program. The new zones would have encompassed areas considered only one-fifth as susceptible to flooding in any given year.

Flood experts, consumer advocates, environmentalists and some insurance experts welcomed the move. But lobbyists for homebuilders, real estate agents and mortgage lenders cautioned lawmakers that the extra cost of flood insurance could hurt housing sales and slow the economy. The program "may need a tune-up, but I don't think it needs radical change," David L. Pressly Jr., president of the National Association of Home Builders, said in an interview.

In mid-November, a senior Republican on the Financial Services Committee, Richard H. Baker of Louisiana, proposed a compromise keeping the current flood zones but requiring coverage for everyone in those areas and in areas adjacent to levees, like those that flooded in New Orleans.

But the committee narrowly adopted an amendment from Rep. Gary G. Miller, a Republican and a former homebuilder from California, which mandated a study of the situation while FEMA put together a clearer picture of the expanded zones.

In a recent interview, Mr. Miller said he had no problem with requiring people in areas like New Orleans to buy the insurance, and perhaps having them pay more for it. But expanding the flood zones, he said, amounted to going to people in, say, Los Angeles and telling them: "We need to raise revenue, so we're going to make you buy into a program that you really are never going to need and never benefit from anyway."

The bill, which the House is expected to take up in the next two weeks, now calls for updating the nation's flood maps, increasing premiums, and reducing subsidies on small businesses and second homes. It would also raise the maximum coverage on a house to $337,000 from $250,000. In all, these changes could bring in perhaps $500 million more a year.

Mr. Frank of Massachusetts said there was "not as much improvement as we'd like, but it's the best we're able to get right now." Lobbyists and legislative aides said they expected no major strengthening of the legislation when the Senate takes it up in coming weeks.

Around the country, opposition to expanding the program seems to be growing. In Michigan, Representative Miller said she was so angry about the program's inequities that she was considering urging that the state withdraw from it. People in her rarely flooded district, she said, had paid four times more in premiums than they had received in claims in the past 10 years.

By contrast, even before Hurricane Katrina, homeowners in just three hurricane-prone states Florida, Louisiana and Texas had received nearly half the money paid by the program since 1978, federal figures show.

Some experts say the program needs to shift from its "one size fits all" approach, which uses the same standards to set rates and the size of flood zones around the nation, to one that takes into account the degree of risk in each area, much as a private insurance plan would. But program officials say they do not believe they have enough history of claims in many areas to make reliable distinctions.

Robert P. Hartwig, the chief economist at the Insurance Information Institute, a trade group, said the shock of Hurricane Katrina had provided "a unique opportunity for quantum reform" of the flood program. But he said the steps now being contemplated "will do relatively little to protect people who live in flood zones and to protect taxpayers around the country."

Looming over the debate are the warnings from meteorologists. Christopher W. Landsea, science and operations officer at the National Hurricane Center, says warmer ocean temperatures and less disruptive wind currents could make the next two or three decades much more active for hurricanes.

And scientists at Colorado State University recently predicted that there could be as many as five intense hurricanes this year. The chance of one striking the Gulf Coast is nearly 50 percent.

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