Senators dug in their heels Tuesday, pushing back against dire warnings from the government's top economic officials of recession, layoffs and lost homes if Congress doesn't quickly approve the Bush administration's emergency $700 billion financial bailout plan.

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Congressional leaders still predicted passage -- with significant changes -- but Wall Street's nerves were hardly soothed. The Dow Jones industrials sank 161 points and now are off more than 500 this week after initially surging on the bailout announcement last week.

Deepening market trouble was just one piece of the economic havoc that Federal Reserve Chairman Ben Bernanke and Treasury Secretary Henry Paulson told senators would ensue if Congress lags in acting on the administration's proposal to rescue tottering financial institutions.

"I share the outrage that people have," Paulson said. "It's embarrassing to look at this. I think it's embarrassing to the United States of America. There is a lot of blame to go around."

But without the bailout plan, Paulson and Bernanke sketched out a dire scenario for senators at a contentious daylong hearing: Neither businesses nor consumers would be able to borrow money, and the world's largest economy would grind to a virtual halt.

In public and in private meetings, both Democrats and Republicans said big changes are needed, presaging a difficult road ahead for the measure.

The legislation the administration is promoting would allow the government to buy bad mortgages and other rotten assets held by troubled banks and financial institutions. Getting those debts off their books should bolster those companies' balance sheets, making them more inclined to lend and easing one of the biggest choke points in the credit crisis. If the plan works, it should help lift a major weight off the national economy that is already sputtering.

Democrats were determined to wrest concessions from the administration on domestic spending and middle-class economic aid. And they said Republicans had to share in the politically tricky task of pushing through a financial bailout six weeks before the elections at a time when millions of everyday Americans are economically strapped.

"It's their problem. It's their bill. And they're going to have to figure out if they can support it," House Speaker Nancy Pelosi, D-Calif., said of Republicans.

"Nobody wants to have to do this," agreed Rep. John Boehner of Ohio, the Republican leader. He said he was hopeful of a quick agreement, despite withering criticism from conservative GOP lawmakers who recoiled at the prospect of federal intervention.

Sen. Jim Bunning, R-Ky., said, "This massive bailout is not a solution. It is financial socialism and it's un-American."

Separately, law enforcement officials said the FBI had begun investigating four institutions whose collapse helped trigger the financial crisis.

The FBI is looking at potential fraud by mortgage giants Fannie Mae and Freddie Mac, Lehman Brothers Holdings Inc. and insurer American International Group Inc., said two officials, speaking on condition of anonymity because of the sensitivity of the investigations. The inquiries, still in preliminary stages, will focus on the financial institutions and the people who ran them, one senior law enforcement official said.

Both parties' presidential candidates also insisted on alterations in the administration's drastic prescription.

Democrat Barack Obama said any plan to rescue Wall Street from its financial woes must ensure that taxpayers are reimbursed and corporate executives are not further enriched for mismanagement. Republican John McCain, too, said the legislation must prevent executives from winning large taxpayer-funded bonuses, and he also said no earmarked spending could be included.

Democrats and Republicans alike demanded that the bailout limit pay packages for executives of companies helped by the rescue.

"Clipping executive compensation is easy right now -- everybody wants it," said Rep. Jack Kingston, R-Ga.

Democrats also were pushing proposals to let the government take some type of stake in the companies that it helps. The administration has balked at that, fearing it would discourage financial companies from getting the help they need through the bailout, thereby blunting the plan's effectiveness.

Democrats also want to let judges rewrite mortgages to lower bankrupt homeowners' monthly payments, another demand the administration is resisting.

Both Sens. Chris Dodd, D-Conn., chairman of the Banking Committee, and the panel's top-ranking Republican, Richard Shelby of Alabama, said significant changes are needed before the rescue plan can be passed. "We have got to look at some alternatives," Shelby said.

Dodd later spoke disparagingly of the administration's proposal.

"What they have sent us is not acceptable," Dodd said.

Dodd said he and other lawmakers understand the need for speedy passage of the measure, but they also want to make sure "we get this right."

Dodd vented his anger earlier at what he called a combination of "private greed and public regulatory neglect" that resulted in an "economic maelstrom."

"We all recognize the gravity of the situation," he said.

Paulson, seated next to Bernanke at the Senate hearing, objected strongly when Chuck Schumer, D-N.Y., asked if $150 billion might be enough to get the program started, with a promise of more to come.

That would be a "grave mistake," and would fail to give the markets the confidence they needed to rebound, Paulson responded.

Rep. Barney Frank, D-Mass., the Financial Services Committee chairman who is leading talks with Paulson on the plan, also called phasing in the bailout "highly unlikely."

Paulson was asked repeatedly why taxpayers should accept the burdens of a bailout.

"You worry about taxpayers being on the hook?" he replied at one point. "Guess what -- they're already on the hook." Paulson suggested that the fallout from the credit crisis would hit almost everyone in the pocketbook unless forceful action was taken. Moreover, the flawed and outdated regulatory system, which didn't catch abuses, needs to be overhauled, he said.

In New York, meanwhile, Bush was telling the U.N. General Assembly that the United States was taking "bold steps" to prevent an economic calamity that would be sure to have major effects around the world.

One of the tricky issues confronting policymakers is how to price the distressed assets that the government would ultimately buy.

Bernanke suggested buying the assets at a "hold-to-maturity" price, which would be based on an estimate of what the securities would eventually be worth as payments came in over the years.

"If the Treasury bids for and then buys assets at a price close to the hold-to-maturity price, there will be substantial benefits," Bernanke said. "First, banks will have a basis for valuing those assets and will not have to use fire-sale prices. Their capital will not be unreasonably marked down."

In contrast, if banks use existing "mark-to-market" rules that require them to value the holdings at what similar securities have recently sold for - in some cases pennies on the dollar -- it could make the whole bailout futile because it would hurt many banks' balance sheets, causing some to fail. "This creates something of a vicious circle," he said.

Berkshire Hathaway To Invest In Goldman Sachs

While Congress debates the merits of the bailout proposal, there some good news for a major Wall Street bank.

Warren Buffett's Berkshire Hathaway corporation is investing at least $5 billion in Goldman Sachs -- a huge vote of confidence for one of the survivors of the crisis that felled two of its investment banking peers. Buffett called Goldman Sachs "an exceptional institution."

In addition to buying $5 billion in preferred stock, Berkshire also got warrants to buy another $5 billion in Goldman's common stock. Goldman also said it will raise another $2.5 billion in its own public stock offering.