Washington - Hoping to stretch a safety net under the nation's tumbling housing market, the Senate voted overwhelmingly on Saturday for final approval of a huge package of legislation that includes an ambitious program to save hundreds of thousands of families from losing their homes to foreclosure.

    The housing legislation is the latest in a series of extraordinary interventions this year by the Bush administration, Congress and the Federal Reserve as they seek to limit the risk that shockwaves in the housing sector will ripple across the American economy and the world financial system. In the process, the central bank and taxpayers have taken on what critics warn are incalculable liabilities and risk.

    The bill grants the Treasury Department broad authority to safeguard the nation's two mortgage finance giants, Fannie Mae and Freddie Mac, potentially by spending tens of billions in federal money to prevent the collapse of the companies, which own or guarantee nearly half of the nation's $12 trillion in mortgages.

    To accommodate the rescue plan for the mortgage companies, the bill raises the national debt ceiling to $10.6 trillion, an increase of $800 billion and the first time that the limit on the government's credit card has grown to 14 digits.

    The Senate, convening for a rare Saturday session as it neared summer recess, approved the bill by a vote 72 to 13. The measure now goes to President Bush, who has said he will sign it, perhaps early next week, to send a reassuring message to the credit markets.

    Lawmakers in both parties hailed the bill as crucially needed. "We are in the midst of the most serious economic crisis to face our nation in many years," said Senator Christopher J. Dodd, Democrat of Connecticut and chairman of the banking committee. "This bill is going to make a difference almost immediately."

    The federal intervention has certainly been bold. The nearly $29 billion loan by the Federal Reserve Bank of New York in March to orchestrate the sale of Bear Stearns to JPMorgan Chase might seem small compared to the Federal Housing Administration's authority, granted in the new legislation, to insure up to $300 billion in refinanced mortgages to help stem a tide of foreclosures.

    Analysts, including the Congressional Budget Office, expect less than $100 billion of that authority to be used. The risk to taxpayers is minimal, analysts say, given higher insurance fees that will be charged to recipients of the refinanced loans.

    And yet, even that $100 billion could seem small compared to the Treasury Department's authority to spend unspecified amounts of tax dollars to rescue Fannie Mae and Freddie Mac if they are in peril of collapse.

    The Treasury secretary, Henry M. Paulson Jr., an architect of the rescue plan, said he expects never to use the new authority. And the Congressional Budget Office predicted that any bailout between now and Dec. 31, 2009, when the authority sunsets, would likely cost $25 billion or less, and that there was a better than even chance of no cost at all.

    But the only real limit on the Treasury's authority is the new $10.6 trillion debt ceiling. There is roughly a $1.1 trillion cushion between the new limit and existing federal debt of $9.5 trillion.

    And naysayers in Congress who voted against the housing bill warned that the government was taking on too much risk, and that government aid would only to reward irresponsibility by corporations and individuals.

    "This bill has moral hazard written all over it," Representative Jeff Flake, Republican of Arizona, said during debate in the House on Wednesday. "We are pretending to chain a monster here and we are, instead, letting that monster loose."

    Indeed, a little-noticed provision in the bill underscores the continuing pessimism about the state of the economy going forward. The provision gives the Federal Deposit Insurance Corporation authority to create so-called bridge institutions for failing savings associations, mirroring a capability that has existed since 1991 for failed banks.

    The new power will give the F.D.I.C. more latitude to continue the operations of savings associations like California-based IndyMac, which failed earlier this month, and buy regulators time to work out a resolution at the lowest possible cost.

    Politics aside, the measure approved by Congress this week represents the most aggressive government intervention in the housing market since the 1989 response to the savings and loan crisis and perhaps the boldest attempt to aid troubled borrowers since the creation of the Home Owners' Loan Corporation in 1933 as part of the New Deal.

    In addition to the program to prevent foreclosures and the rescue plan for Fannie Mae and Freddie Mac, the 694-page bill contains a sweeping new regulatory structure for the mortgage giants, including the creation of an independent regulator, a stand-alone federal agency with a director appointed by the president and confirmed by the Senate.

    The bill also includes $15 billion in housing related tax incentives, including a $7,500 tax credit for first-time homebuyers and business tax breaks for home builders and other large corporations.

    There is also an array of items buried deep in the legislation, and the implications of some of them are not yet clear. There are provisions, for example, that grant or extend Section 8 federal housing subsidy eligibility to residents of specific properties in Malden, Mass. and San Francisco. And one business tax incentive seems intended for a specific but unidentified automobile manufacturer "that will produce in excess of 675,000 automobiles" between Jan. 1 and June 30, 2008.