The Washington region’s middle class would be one of the groups hardest hit by a House Republican proposal to cut in half the top mortgage interest deduction used by American homeowners.

The proposal, unveiled Thursday, says homeowners would be able to deduct interest payments made on only their first $500,000 worth of home loans. Under the current law, homeowners can deduct interest on mortgages of up to $1 million.

“Personally, I think it would impact the middle class drastically,” said Nicholas Lagos, a real estate broker at Century 21 Gawen Realty in Arlington. “I don’t think it would have much of an effect on the upper class, the people in the $3 million and $4 million homes. It’s not going to impact them one bit. The people who are your regular middle-class people, . . . it will impact them.”

Prices are high in the Washington area — for example, the median sale price for a home in Bethesda is $872,500, according to ShowingTime RBI, based on listing activity from Bright MLS. That means many homeowners take out substantial mortgages when buying their homes.

“If you think about a $1 million mortgage,” said Laly Kassa, managing director at Chevy Chase Trust, “in the initial years, that interest expense is in the $30,000 range. Having that capped is going to have an impact. . . . For the younger clients, there it will have a really significant impact. It’s like losing a third of that deduction. For younger families, there it will impact what they buy and what they spend their discretionary funds on. That’s $10,000 out the door right at the inception of a mortgage.”

ATTOM Data Solutions, a real estate data and analytics firm, found that the District, Maryland and Virginia are among the 10 jurisdictions with the highest percentage of mortgages above $500,000. In the District, 35 percent of all mortgages are for more than $500,000. In Maryland, 9 percent of mortgages are for more than $500,000; in Virginia, 8 percent are.

Loudoun County has the largest average home mortgage deduction in the country, according to an August report by the Tax Foundation. Homeowners in Loudoun County who itemize their deductions deduct an average of $6,365 on their income taxes. Six of the top 10 areas for average home mortgage interest deductions are in the Washington region. They are Loudoun, Falls Church ($5,450), Fairfax County ($4,805), Calvert County ($4,677), Stafford County ($4,667) and Howard County ($4,656).

“What that proposal is not doing is it is not making adjustments for areas that are not necessarily affluent but expensive areas,” Lagos said.

Some Republicans argued that other tax breaks would offset the loss of the mortgage interest deduction. The tax plan proposes, for instance, an elimination of the alternative minimum tax and a lowering of the individual tax rates.

House Ways and Means Committee Chairman Kevin Brady (R-Tex.) noted that because the GOP’s tax plan also proposes to double the standard deduction for couples to $24,000 and revise individual income brackets, “there is tax relief for Americans regardless of where they live.”

But Chevy Chase-based broker Hans Wydler of Wydler Brothers Real Estate questions the motivation of the cap.

“For a long time, the government encouraged homeownership,” Wydler said. “Multiple presidents on both sides of the aisle talked about how important it is. It’s the American Dream. This is against that, which is fine, if they’re changing what the objective is. If the American Dream, what they defined it before is no longer relevant, that’s fine. . . .  Just articulate it. But they’re saying, ‘Yes we want the American Dream,’ but they are attacking it economically. It’s just sort of bizarre.”

The bill also would limit the amount homeowners are allowed to deduct in local property taxes from their taxable income. The House Republican proposal caps this deduction at $10,000.

“The benefit of the mortgage interest deduction goes away for most people over time,” said Danielle Hale, chief economist at Realtor.com. “But the property tax benefit continues year after year.” She said that “most property tax rates in this area are 1 percent or higher,” meaning a $10,000 cap would affect people with home values of $1 million or more. “That is going to affect a much larger share of people in this area. The tax savings you get from taking a property tax deduction is not quite as large generally as the mortgage interest deduction, but it will still be noticeable.”

The National Association of Realtors and the National Association of Home Builders have vowed to use their lobbying power to save the mortgage interest deduction.

“That’s a very important deduction, one of the few deductions we have left,” Lagos said. “That is an important subsidy to a homeowner, especially to a dual-income family with children or anyone who is buying in this area. . . . I really think in this particular case that they need to focus on continuing to allow the middle class to continue to prosper instead of penalizing them for living in metropolitan areas.”