South By North Strategies, Ltd.

The New Republic points out that the mortgage-interest deduction is a broad middle-class tax break only if the middle class is defined as the top 20 percent of households.

If we get rid of the mortgage-interest deduction, about 22 percent of people in the 40^ththrough 60^th income percentiles will pay higher taxes —on average $215 a year, or about half a percent of current after-tax income. For those in the 60^th through 80^th percentiles, only 45 percent would see their taxes go up—by an average $689, about 1 percent of after-tax income. While there are tax hikes, they’re fairly small in relative terms, and what’s more, less than half of people in these income groups would have to pay them. And below the 40^th percentile, almost no one would be affected by repealing the mortgage-interest deduction.
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The impact is more substantial farther up the earnings ladder. In the 90^th through 95^thpercentiles, for example, 74 percent of people would see their taxes rise, costing them an additional $2,643 per year on average, about 1.75 percent of their after-tax income. It’s true that repealing the deduction would (in relative terms) have only a small impact on the top 1 percent of earners, but that’s largely because, as AaronW noted in the comments thread, “for the truly wealthy, mortgage debt is a much smaller proportion of their income than for middle income earners.”