Ben S. Bernanke, right, the Federal Reserve chairman, and Treasury Secretary Henry M. Paulson Jr. are working to halt the upheaval.

This article was reported by Peter Baker,Stephen Labaton and Eric Lipton and written by Mr. Baker.

WASHINGTON - For the last year, as the American economy lurched from crisis to crisis, the chairman of the United States Federal Reserve, Ben S. Bernanke, had been warning Henry M. Paulson Jr., the United States Treasury secretary, that the worsening situation might ultimately force a sweeping intervention by the federal government.
A longtime student of the Great Depression, Mr. Bernanke was acutely aware of what could happen without a decisive move. Finally, the moment that called for action arrived late on Wednesday, September 17. Less than 24 hours after the Federal Reserve had bailed out American International Group, the giant insurer, it was clear that the turmoil gripping Wall Street was only growing worse and that ad hoc solutions were not working.
Talking into a speaker phone from his ornate office, Mr. Bernanke told Mr. Paulson that it was time to adopt a comprehensive strategy that the United States Congress would have to approve. Mr. Paulson understood. Previously reluctant to send Congress a plan that lawmakers had warned had little chance of quick passage, he had worried that a rejection would only further shock the markets. But during two conference calls on that Wednesday night and Thursday morning, he agreed that they had no choice.
“It just happened dramatically,” Mr. Paulson said in an interview on September 19. “There was only one way that we could reassure the markets and deal with a very significant and broad-based freezing of the credit market. There was no political calculus. It was overwhelmingly obvious.”
Just like that, Mr. Bernanke, the reserved former Ivy League professor, and Mr. Paulson, the hard-charging former Wall Street deal maker, launched what would be the United States government’s largest economic rescue operation in modern times, one that rivals the Iraq war in cost and may redefine Washington’s role in the marketplace for years.
The plan to buy $700 billion in troubled assets with taxpayer money was shaped by two men who did not know each other until two years ago, but now find themselves brought together by history. If Mr. Bernanke is the intellectual force and Mr. Paulson the action man of this unlikely tandem, they have managed to create a nearly seamless partnership as they rush to stop the financial upheaval .
Along the way, they have cast aside the administration’s long-held views about regulation and government involvement in private business.
“There are no atheists in foxholes and no ideologues in financial crises,” Mr. Bernanke told colleagues at one meeting, according to a participant.
The improvisational nature of their effort has turned President Bush and Congressional Democrats into virtual bystanders, sometimes uncertain about what comes next.
Mr. Bernanke took office in February 2006 and Mr. Paulson five months later, both Republicans and Bush appointees, yet arriving from starkly different places. Mr. Bernanke, 54, had developed a conciliator’s style as chairman of Princeton University’s economics department. Mr. Paulson, 62, rose to the top of the investment bank Goldman Sachs by working the phones and pounding the occasional table.
“Hank is just the most hyperactive, getit- done kind of guy who’s always trying to get the problem solved and move on. He’s impatient to fix things,” said Allan B. Hubbard, a former national economic adviser to Mr. Bush.“Ben is much more low-key. He’s very thoughtful. He’s an incredible thinker, listens well, analyzes well and is not intimidated by anyone. It’s probably a great pair.”
The Hammer, as Mr. Paulson has been called since he played football at Dartmouth College, brought to Washington his characteristic intensity. “He is a hurricane. He is used to living in a turbulent world,” said John H. Bryan Jr., a friend and former chief executive of the Sara Lee Corporation, the consumer-goods company.
Mr. Bernanke has a more obscure nickname, Helicopter Ben, after a speech he gave in 2002 in which he talked about the Feds helicopter drops” of emergency money to keep the system liquid.
For Mr. Bernanke, the current crisis is the culmination of a lifetime of figuring how the system works from a theoretical viewpoint. He knew long ago that he might someday be called on to act on his studies. Vincent R. Reinhart, a former Fed official, said Mr. Bernanke’s research into Japan’s financial crisis in the 1990s had reinforced his view that government had to be aggressive in market crises.
Mr. Paulson said Mr. Bernanke had long warned that a moment might come like the one they saw recently. “Going back a long time, maybe a year ago, Ben, as a worldclass economist, said to me, ‘When you look at the housing bubble and the correction, if the price decline was significant enough,’ ” the only solution might be a large-scale government intervention, he said.
Mr. Paulson said he agreed but hoped it would not come to that. “I knew he was right theoretically,” he said. “But I also had, and we both did, some hope that, with all the liquidity out there from investors, that after a certain decline that we would reach a bottom.”