Of all the deductions woven into the sprawling U.S. tax code, few have been more fiercely guarded than the enormous tax break that lets homeowners deduct the interest they pay on their mortgages.

But as Congress and the White House negotiate the first major rewrite of tax laws in decades, changing the generations-old mortgage-interest deduction — which costs the government roughly $100 billion a year — has gone from far-off possibility to part of the conversation.

The outcome of that debate could have profound long-term effects on homeowners across the country — and particularly those in the Washington area, who tend to benefit from the tax break more than many other Americans due to the region’s hefty home prices and high incomes.

As the Obama administration and lawmakers on Capitol Hill scramble to defuse automatic spending cuts and tax increases set to take effect Jan. 1, a herd of sacred cows — from Social Security and Medicare to deductions for charitable giving and mortgage interest — are in danger of losing their untouchable status.

Members of both parties have largely steered clear of detailed proposals so far. But plans put forth in the past year by President Obama and Mitt Romney to place limits on annual total tax deductions are likely to crimp the mortgage-interest deduction for certain taxpayers. Top congressional Republicans also have expressed openness to limiting total tax deductions as part of an overall budget deal. In addition, the presidentially appointed Simpson-Bowles fiscal commission suggested scaling back the mortgage-interest deduction as part of its own set of tax-related proposals.

Current law allows homeowners to deduct the interest paid on mortgage balances up to $1 million, including on second homes, as well as on $100,000 worth of home-equity loans. The deduction overwhelmingly benefits wealthier families, partly because they tend to have larger mortgages and pay more interest, and partly because most low- and middle-income Americans do not itemize deductions on their tax returns. It also tends to favor homeowners on the East and West Coasts, as well as those in large cities such as Chicago, where average home prices are higher.

Edward Kleinbard, a tax expert and law professor at the University of Southern California, said the mortgage-interest deduction represents the kind of government “extravagance” that the country no longer can justify, given its fiscal troubles.

“We simply cannot afford wasteful government subsidy programs anymore, and this is one of the most important examples of that,” Kleinbard said. “It’s very much a subsidy to those Americans who need it least.”

True enough, said Moody’s chief economist Mark Zandi, but the deduction nevertheless has become ingrained in the psyche of home buyers over generations, and reducing it would have real effects.

“It’s a very visceral thing for people,” Zandi said. “People account for it when they think about how much house they could afford to buy. You take that away, and house prices are going to weaken. They are going to decline.”

He said it is possible that any price declines could prove minor over the long term, and that the housing industry itself ultimately would benefit if the country were on a firmer fiscal footing.

“I think we’re going to get tax reform, and the [mortgage-interest deduction] is going to be part of it,” Zandi said. But he said any changes should be phased in over a period of years.

Neither the White House nor lawmakers have shown an inclination to scrap the deduction, which has been around in some form since 1913. Unlike some other deductions, it remained intact in the last major tax overhaul, in 1986. It is the most high-profile housing-related tax incentive, but not the only one. For instance, homeowners can deduct their local property taxes and are exempt from some capital gains taxes when selling a residence if they have lived there long enough.

Some researchers have argued that the tax break actually does little to boost home ownership, given that people affluent enough to benefit from the deduction are likely to buy homes even without it.

“It’s a clunky subsidy for home ownership,” said Ted Gayer, co-director of the economic studies program at the Brookings Institution. “It also subsidizes things we don’t want to subsidize, like borrowing a lot of money for your home. . . . It’s a tax credit for people who have large mortgages.”

Any efforts to shrink the mortgage-interest deduction are likely to face stiff opposition from real estate agents, home builders and others who argue that millions of middle-class Americans also benefit. Few special-interest groups are better equipped to wage an all-out campaign than the real estate industry, which has defended the tax break for decades as a driver of home construction and a key motivator of home ownership.

“It has always been NAR’s position that the [mortgage-interest deduction] is vital to the stability of the American housing market and economy,” Gary Thomas, president of the National Association of Realtors, said in a statement. “And we will remain vigilant in opposing any future plan that modifies or excludes the deductibility of mortgage interest.”

David H. Stevens, chief executive of the Mortgage Bankers Association, said many people in his industry expect that the mortgage-interest deduction will be part of the long-term debate over fixing the nation’s budget woes, and he acknowledged that it could face changes as part of an overhaul of the tax code.

But what he and other industry advocates don’t want, Stevens said, are sudden or drastic
changes to current policy as part of a last-minute effort to avert the “fiscal cliff” on Jan. 1. He said such a move would threaten the fledgling housing recovery, cause harm to the larger economy and put a dent in the pocketbooks of many middle-class Americans.

“We are by no means knee-jerk, reactionary on this subject,” Stevens said. “The greatest concern we have is that in a rush to deal with the fiscal cliff before the year’s end, it could lead to an environment where less-thoughtful decision-making could occur. . . . This is something that has to be managed extremely carefully. This could be stepping over dollars to save pennies if we do it too soon.”