As Washington searches for ways to drum up tax revenues in a bid to avert the fiscal cliff, the interest deduction some homeowners claim on their mortgages may be on the chopping block. The possibility that this deduction might go away has prompted the sort of dire warnings that might be reserved for news that the American flag will lose its stars and stripes: The housing market, finally recovering from a long decline, will plunge anew as values fall. People won't buy as many homes. Middle-class families will suffer and despair. But this view, voiced with the most conviction by...