The FHA could insure $300 billion in such mortgages, which would be available to homeowners who showed they could afford a new loan. Banks would first have to agree to take a large loss on the existing loans in exchange for avoiding an often-costly foreclosure.

The plan also is designed to relieve a broader credit crunch that has taken hold because of rising defaults and falling home values. To free up safer and more affordable mortgage credit, the bill permanently would increase to $625,000 the size of home loans that Fannie Mae and Freddie Mac can buy and the FHA can insure. They also could buy and back mortgages 15 percent higher than the median home price in certain areas.

It goes far beyond addressing the current crisis, however.

The legislation overhauls the Depression-era FHA. It requires lenders to show how high a borrower's payment could get under the terms of his mortgage. It provides $180 million in pre-foreclosure counseling for struggling homeowners.

The Treasury Department gains unlimited power, until the end of 2009, to lend money to Fannie Mae and Freddie Mac or buy their stock should they need it. The Federal Reserve takes on a new "consultative" role overseeing the companies.

The measure includes $15 billion in tax cuts, including a significant expansion of the low-income housing tax credit and a credit of up to $7,500 for first-time home buyers for houses purchased between April 9, 2008, and July 1, 2009.

Democratic leaders, recognizing that the measure could be one of the last items to become law during what's left of their abbreviated election-year schedule, tacked on an $800 billion increase, to $10.6 trillion, in the statutory limit on the national debt.

Conservative Republicans were vehemently opposed to the bill, particularly the help for Fannie Mae and Freddie Mac. Critics charge the companies enjoy lavish profits in good times and wield their outsized political clout to resist regulation while depending on the government to bail them out should they falter.