IT’S A TRUISM that the U.S. economy cannot fully recover as long as millions of homeowners continue to owe more money on their houses than the property is worth. But if the past three years have proven anything, it is the difficulty of designing a housing rescue with benefits that outweigh the costs.

Nevertheless, in his address last week to a joint session of Congress, President Obama pledged to give it another try: “To help responsible homeowners,” he declared, “we’re going to work with federal housing agencies to help more people refinance their mortgages at interest rates that are now near 4 percent. That’s a step . . . that can put more than $2,000 a year in a family’s pocket and give a lift to an economy still burdened by the drop in housing prices.”

Would this push for government-backed refis succeed where others have failed? Mr. Obama offered few details, in part because the concept is still being hashed out between his administration and skeptical regulators at the Federal Housing Finance Agency, which controls the two bailed-out mortgage agencies — Fannie Mae and Freddie Mac. But he does not seem to be contemplating the wholesale, multibillion-
dollar refinancing effort that some economists have recommended.

The far likelier approach is to loosen eligibility criteria for an existing program, the Home Affordable Refinance Program. Known as HARP, the program is supposed to spread the benefits of the Federal Reserve’s low-interest policies to homeowners who are “underwater” but still making their mortgage payments. So far, HARP has helped 800,000 homeowners whose loans are backed by Fannie and Freddie get into lower-rate mortgages. The results have been modest, in part because banks are reluctant to redo loans that are already performing and in part because Fannie and Freddie charge stiff fees. As many as 2.9 million borrowers might benefit from a more aggressive program, according to a recent projection by Congressional Budget Office (CBO) economists, resulting in 111,000 fewer foreclosures and a $7.4 billion boost to disposable income in the first year.

That wouldn’t shore up the housing market as such, but it might provide a modest fillip to consumer spending and, by extension, the economy generally. Who would pay? Mostly holders of mortgage-backed securities (MBSes), who would be obliged to accept face value for bonds that currently trade at a premium. The CBO economists say the hit to investors could be as much as $15 billion. But there’s nothing illegal or improper about that — MBSes are always subject to prepayment. And to some extent, this would represent the recovery of a windfall that bondholders reaped over the past two years, when market instability blocked refis that otherwise would have taken place, given falling interest rates.

The taxpayer would be on the hook, too, since the U.S. Treasury, the Federal Reserve and Fannie and Freddie themselves hold a lot of MBSes. But some of the government’s cost might be offset if fewer homeowners end up defaulting.

Basically what we have here is a plan — if it works at all — that would move money around within the economy, shifting it from bondholders and taxpayers to a select portion of underwater homeowners. How much more net spending on goods and services would occur is a wide-open question.

While the losses to bondholders would take place immediately, the gains to homeowners would occur in monthly installments over several years. Probably homeowners would use at least some of their mortgage savings to pay down other debts, such as credit cards and college loans. Dramatic stimulus? No. A modest aid to repairing household balance sheets? Maybe.