University of Maryland economics professor Peter Morici misrepresented the Republican's proposed change in the mortgage interest deduction in a debate with my friend Jared Bernstein on Morning Edition. Morici said that the proposed cap would only hit people paying more than $500,000 in interest on their mortgage. In fact, it would cap the amount of principal on which interest could be deducted at $500,000.
Morici is correct that this would hit very few people, since it means having an outstanding balance on a mortgage of more than $500,000. Furthermore, the cap only applies to the margin over $500,000. This means that someone with outstanding principal of $540,000 would still be able to deduct the interest on $500,000 or more than 90 percent of their interest payment.
It is only the interest on the last $40,000 that would no longer be deductible. If they are paying 4.0% interest on their mortgage this would mean they are missing a deduction of $1,600, which translates into a tax increase of $400 for someone in the 25 percent tax bracket.
Budget Geek reminds me in a comment below that current mortgages are grandfathered so they would still be able to deduct interest on principal in excess of $500,000. (There is already a cap at $1 million.) It is only newly issued mortgages that would be subject to the $500k cap.