Not so now. As Congress grapples with dealing with the debt ceiling, limiting the deduction--which "costs" the Treasury more than $80 billion a year--is definitely on the table. The most discussed changes would either reduce the deduction to the first $500,000 of mortgage principal (the current limit is $1 million) or, more modestly, allow the interest on principal above $500,000 to be deducted against a maximum 28% tax rate. The more drastic change would raise total tax revenues up to $600 billion over the next 10 years, according to one estimate.
The Harvard Joint Center for Housing Studies estimates that limiting the deduction would cost the average tax payer $2400 year. But, as the chart shows, the increased tax burden falls disproportionately on high-income tax payers who own expensive homes. Targeting high-income consumers gives Congress political cover. At the same time, it allows economists to argue that: one, a tax increase of $400 to $500 a month wouldn't much affect the home buying behavior of the top one percent of American consumers; and, two, that most new home buyers, who have moderate incomes, would be largely unaffected by limiting the deduction.
Housing lobbyists will argue that the deduction is an important underpinning of the American dream of homeownership. We'll probably know before year's end if that mom and apple pie argument carries the day.
P.S. The homeownership rate in the U.S. is about 64%. At least 15 other countries have a higher homeownership rate (topped by Bulgaria at 97%), and among them only the Netherlands and Switzerland have a straightforward tax deduction for mortgage interest.