Still, the move will likely start an intense political debate during the presidential election campaign over who is to blame for the financial crisis that prompted the rescue.

Image People lined up Tuesday outside an office of an American International Group subsidiary in Singapores main business district. Credit... Vivek Prakash/Reuters

Representative Barney Frank, Democrat of Massachusetts and chairman of the House Financial Services Committee, said Mr. Paulson and Mr. Bernanke had not requested any new legislative authority for the bailout at Tuesday night’s meeting. “The secretary and the chairman of the Fed, two Bush appointees, came down here and said, ‘We’re from the government, we’re here to help them,’ ” Mr. Frank said. “I mean this is one more affirmation that the lack of regulation has caused serious problems. That the private market screwed itself up and they need the government to come help them unscrew it.”

House Speaker Nancy Pelosi quickly criticized the rescue, calling the $85 billion a "staggering sum." Ms. Pelosi said the bailout was "just too enormous for the American people to guarantee." Her comments suggested that the Bush administration and the Fed would face sharp questioning in Congressional hearings. President Bush was briefed earlier in the afternoon.

A major concern is that the A.I.G. rescue won’t be the last. At Tuesday night’s meeting. lawmakers asked if there was any way of knowing if this would be the final major government intervention. Mr. Bernanke and Mr. Paulson said there was not. Indeed, the markets remain worried about the financial condition of major regional banks as well as that of Washington Mutual, the nation’s largest thrift.

The decision was a remarkable turnaround by the Bush administration and Mr. Paulson, who had flatly refused over the weekend to risk taxpayer money to prevent the collapse of Lehman Brothers or the distressed sale of Merrill Lynch to Bank of America. Earlier this year, the government bailed out another investment bank, Bear Stearns, by engineering a sale to JPMorgan Chase that left taxpayers on the hook for up to $29 billion of bad investments by Bear Stearns. The government hoped at the time that this unusual step would both calm markets and lead to a recovery by the financial system. But critics warned at the time that it would only encourage others to seek bailouts, and the eventual costs to the government would be staggering.

The decision to rescue A.I.G. came on the same day that the Fed decided to leave its benchmark interest rate unchanged at 2 percent, turning aside hopes by many on Wall Street that the Fed would try to shore up confidence by cutting rates once again.

Fed and Treasury officials initially turned a cold shoulder to A.I.G. when company executives pleaded on Sunday night for the Fed to provide a $40 billion bridge loan to stave off a crippling downgrade of its credit ratings as a result of investment losses that totalled tens of billions of dollars.