Reed said he and his 11 siblings inherited the lake cottage in his district from his mother. That arrangement didn’t work well, and he borrowed money to buy them out. He now owes between $100,000 and $250,000 on the cottage and between $50,000 and $100,000 on his primary home in Corning, where he was mayor before coming to Congress in 2010.

“This longstanding tradition is something that if we move away from we should do it very carefully,” he said. “And we should do it in a very well-thought-out manner.”

Largest Breaks

The mortgage-interest deduction, with an estimated cost of $72 billion in forgone revenue in 2014, is one of the largest tax breaks in the Internal Revenue Code and the subject of a real-estate industry lobbying campaign to protect it.

Taxpayers can deduct interest on mortgages of up to $1.1 million on as many as two homes, a “main home” where they live most of the time and a second home. At least for voting purposes, lawmakers declare their primary residences in their home states. A rule that would constrain the deduction to primary residences would limit the break.

The Internal Revenue Service doesn’t require taxpayers to break out their mortgage interest by home, and the agency doesn’t have data on the cost of the break. The nonpartisan Tax Policy Center offers a rough estimate that repealing the deduction could generate $8 billion a year for the government.

The second-home break was one of the few that Mitt Romney, the 2012 Republican presidential candidate, suggested could be ended to pay for lower tax rates. It’s one of the specific ideas lawmakers offer when asked what breaks should disappear.

‘Heavy Lobbying’

“If you want to know why it’s there, it’s there because of heavy lobbying,” said Dennis Ventry, a tax law professor at the University of California, Davis, who cites the influence of lawmakers from resort areas and not any real benefit in promoting homeownership. “So much of our homeownership subsidy has this component to it, this visceral symbolic component.”

Jamie Gregory, deputy chief lobbyist for the National Association of Realtors, said the “light bulb finally goes off” for lawmakers when they’re presented with data showing that 900 of the approximately 3,100 counties in the U.S. have more than 10 percent of their housing stock as second homes, according to the realtors’ group.