WASHINGTON—The Obama administration and federal regulators are reversing course on some of the biggest postcrisis efforts to tighten mortgage-lending standards amid concern they could snuff out the fledgling housing rebound and dent the economic recovery. On Tuesday, Mel Watt, the newly installed overseer of Fannie Mae and Freddie Mac, said the mortgage giants should direct their focus toward making more credit available to homeowners, a U-turn from previous directives to pull back from the mortgage market. In coming weeks, six agencies, including Mr. Watt's, are expected to finalize new rules for mortgages that are packaged into securities by private investors....