WASHINGTON — Of all the deductions woven into the sprawling US 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.
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 would likely 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.
Current law allows homeowners to deduct the interest paid on mortgage balances up to $1 million, including on second homes, and 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 their taxes. 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.’’
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.
And 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.