Perhaps the most sacred of all the sacred cows in the tax code, the home mortgage deduction has long been seen as critical to a major element in the American dream — owning your own home. It’s also a boon to home builders, construction workers, the financial services industry and local governments that benefited from fatter real estate tax revenue. But after nearly a century, the mortgage deduction may face a day of reckoning. Though out of the spotlight for the moment while the lame-duck Congress thrashes to an end, the mortgage deduction issue is likely to resurface next year when new representatives — including a lot more deficit-hawk Republicans — take their seats.