Washington ? That most sacred of tax breaks, the mortgage interest deduction that has helped millions buy homes, could vanish if President Bush and Congress follow the recommendations of his tax advisory board.

Nine tax experts, tasked with developing simpler and fairer tax laws, concluded that the deduction does more for wealthier taxpayers than for people struggling to buy a home. But mortgage bankers and real estate agents see irreparable harm if the tax break disappears.

The National Association of Realtors estimated that such a change could cut home prices by 15 percent, bad news for owners who have seen the value of their homes increase.

“You’re going to be taking away from Middle America,” said David Lereah, the association’s chief economist. “Everyone, whether you use the mortgage interest deduction or not, the value goes down. You’ve just reduced the retirement nest egg for everyone.”

The current tax break lets homeowners deduct interest paid during the year on a mortgage up to $1 million and a home equity loan worth up to $100,000. Homeowners also benefit from breaks that let taxpayers deduct state and local property taxes from the federal bill.

Nearly 36 million taxpayers claimed the deduction in 2003, according to the most recent statistics compiled by the Internal Revenue Service.

The President’s Advisory Panel on Federal Tax Reform urged the administration to do away with the deduction and replace it with a credit worth 15 percent of interest paid during the year. They would scrap the deduction for property taxes, too.

Mortgages eligible for the tax break would be limited by a formula reflecting the average regional price of housing. If in place today, that range would spread from $227,000 to $412,000. Mortgages for second homes and interest paid on home equity loans would not be eligible for the credit.

Taxpayers who currently own homes would have five years before they had to use the new credit. During that period of transition, a taxpayer still could take a deduction but the size of the mortgage eligible for a tax break gradually would fall. At the end of five years, everyone would be using the proposed credit.

Connie Mack, a former Florida senator and chairman of the tax panel, said that fewer than 5 percent of mortgages in the nation exceed the proposed cap.

“It is a fair plan,” he said. “It shares the benefits.”

For homeowners with a small mortgage who don’t itemize their deductions, the credit means a new tax benefit defraying the cost of housing.

Taxpayers who bought $1 million homes expecting a generous tax break could be in for a shock, said Michael Fratanponi, senior director of single family research and economics at the Mortgage Bankers Assn.

“That’s going to really bite,” he said.

Greg McBride, senior financial analyst at Bankrate.com, said the proposed credit wouldn’t help homeowners in regions of the country, like New York and California, where housing prices have skyrocketed.

“It seems to ignore the plight of a first-time buyer in an expensive market,” he said.

Because the panel would convert the deduction to a credit, taxpayers who pay income tax at marginal rates over 15 percent would see their benefits shrink.

Clint Stretch, director of tax policy for Deloitte Tax, calculated that housing would get more expensive, for example, for a family carrying a $500,000 mortgage and earning income in the 25 percent tax bracket. The proposal would take away $4,400 of the tax benefit.

Stretch said the shock might be as much psychological as financial.

“It also has a piece of American dream about it,” he said. “It’s not just what people get now, but what they hope and dream they’re going to have someday. I think a lot of taxpayers who would never have a mortgage above the (limits) sure hope they would.”