At the heart of the President Barack Obama's ambitious plan to rescue the housing market is the conviction that restructuring distressed mortgages will keep struggling borrowers in their homes and help insert a floor beneath plummeting property values. With $75 billion dedicated to reworking troubled loans, that's a big bet—especially considering that a top banking regulator said last December that almost 53 percent of loans modified in the first quarter of 2008 went bad again within six months. But supporters argue that mortgage modifications need to be properly engineered to work—and many early ones weren't. To that end, the Obama administration on Wednesday unveiled fresh details on its plan to restructure at-risk loans and help as many as four million home owners avoid foreclosure.