Early in the morning and out of public view, the president signed it without fanfare in the Oval Office, adding his signature to a measure he once threatened to veto. The White House said he was accompanied by Treasury Secretary Henry Paulson, Housing and Urban Development Secretary Steve Preston and other administration officials.

"We look forward to put in place new authorities to improve confidence and stability in markets," White House spokesman Tony Fratto said. He said the Federal Housing Administration would begin to put in place new policies "intended to keep more deserving American families in their homes."

The legislation is regarded as the most significant housing bill in decades. It won approval from lawmakers eager, in an election year, to come up with an answer to the growing housing crisis.

The measure includes $300 billion in new loan authority for the government to back cheaper mortgages for troubled homeowners; $3.9 billion for communities to fix up foreclosed properties causing blight in neighborhoods; and $15 billion in tax cuts, including an expanded low-income housing tax credit and a credit of up to $7,500, to be repaid, for some first-time home buyers.

The number of homeowners who could lose their homes to foreclosure by the end of 2009 is estimated by some to be around 2.8 million. Under the legislation, 400,000 having trouble with payments could avoid it by trading their loans for new, more affordable mortgages through the Federal Housing Administration.

The White House sought to focus attention on parts of the legislation aimed at calming markets. Those include the offer of a temporary but unlimited government line of credit for troubled government-sponsored mortgage companies Fannie Mae and Freddie Mac. The Treasury Department gains power, until the end of 2009, to lend them emergency money or buy their stock.