U.S. share prices were stable on Wednesday after sliding sharply on Tuesday, reflecting disappointment that Geithner's much-awaited plan lacked detail, both on dealing with badly weakened banks and on avoiding home foreclosures. Asian markets were down Wednesday while European indexes ended mixed.

As Geithner testified before the Senate Budget Committee, whose members clearly were sobered by the magnitude of the crisis and of the administration's proposed response, the committee's chairman suggested that the Treasury Department might have to return to Congress for an additional $300 billion to $500 billion to help stabilize the financial sector.

"I don't think the right answer is zero for the housing crisis and the financial sector," said the chairman, Senator Kent Conrad, Democrat of North Dakota.

Geithner acknowledged the concerns about how markets had reacted to his first major public pronouncement as Treasury secretary.

"I completely understand the desire for details and commitments," he said. But, alluding to the troubled first portion of the federal bailout plan known as TARP, he said, "we're going to do this carefully" and "get it right."

He said that a more detailed announcement on helping homeowners was weeks away - and would probably come from Obama himself. But he provided almost no specifics or figures on that or other matters, though he spent considerable time trying to bat down grim-sounding numbers thrown at him by worried senators of both parties.

When Senator Lamar Alexander, Republican of Tennessee, asked him about the estimate of some economists that U.S. banks might hold $2 trillion in bad assets - and asked Geithner whether he had not underestimated the problem - the Treasury secretary neither confirmed nor denied the number.

Still, he assured committee members that the delays in providing a fuller picture did not mean a half-hearted approach.

"We believe the policy response has to be comprehensive and forceful and that there is more risk and greater cost in being gradualist and tentative than there is in aggressive action," he said.

Geithner said the administration had learned from failed policy initiatives at the outset of the Great Depression, and during the early years of the Japanese recession of the 1990s, adding, "We believe that the action has to be sustained until recovery is firmly established."

More than one senator asked the Treasury chief about media estimates that the government had now taken on outlays, commitments or loan guarantees of a staggering $7.8 trillion.

Geithner went to great pains to note that the ultimate cost would be far less.

"I understand that," said Senator Jeff Sessions of Alabama, a prominent Republican on the panel. "And you're saying, we don't want to be tentative and we don't want to be limited. But I don't want to be reckless, overspending and unprincipled, either. And so there's a tension here."

A central pillar of the plan that Geithner announced Tuesday would create one or more so-called bad banks to buy and hold banks' bad assets, using a mix of taxpayer and private money.

Few details on this were provided, and intense, behind-the- scenes talks in the White House on the best way to do this continued.

The administration and the U.S. Federal Reserve are also proposing to expand a lending program that would spend as much as $1 trillion to make up for the even greater decline in the issuance of securities backed mainly by consumer loans.

Banks would also be given new infusions of capital with which to lend. In exchange, they would have to cut the salaries and perks of their executives and sharply limit dividends and corporate acquisitions.

But the initial reaction from the markets, lawmakers and economists was sharply negative.

Some Wall Street experts said the plan relied too greatly on the same vague solutions proposed under the Bush administration.

The Dow Jones industrial average fell more than 4 percentage points Tuesday.

Asked late Tuesday about the stock market reaction, Obama told ABC News, "Wall Street, I think, is hoping for an easy out on this thing and there is no easy out. Essentially what you've got are a set of banks that have not been as transparent as we need to be in terms of what their books look like."

Obama, who has traveled this week to Indiana, Florida and Virginia to make his case for quick action on an $800 billion stimulus plan to a deeply worried public, said that he was constantly seeking the right balance in his comments on the crisis.

"I'm constantly trying to thread the needle between sounding alarmist but also letting the American people know the circumstances that we're in," he said. "And the fact of the matter is, is that we are in not just an ordinary recession, we are in a perfect storm of financial problems."

Geithner, with a formal speech, live television interviews and then his Senate appearances Tuesday and Wednesday, announced the overhaul of the bank bailout program of the administration of George W. Bush, which had proved largely ineffective at getting credit flowing again.

The changes are intended to ensure that the $350 billion still available from the original $700 billion allocated by Congress will accomplish what the Bush team's effort did not: restart lending, clean up the balance sheets of banks, provide relief to housing markets and restore confidence in the government's response.

Tuesday also was the public debut of Geithner, the man Obama has called "the chief economic spokesman for my administration."

Geithner, 47, has now taken full responsibility for issues on which Lawrence Summers, his boss at the Treasury in the 1990s and now the chief economic adviser in the White House, had filled in while Geithner weathered a confirmation controversy over unpaid taxes.

In his testimony Tuesday, Geithner told senators that he wanted to address the public's desire for a clearer idea of the new administration's thinking but that he wanted to get the details right - and not repeat what he has called "the zigs and zags" and mistakes of the past.

Among those was former Treasury Secretary Henry Paulson Jr.'s redefinition of the bailout program after he had sold it to Congress, a shift that unsettled financial markets further.

But Geithner has also come under fire, both for his role in the miscalculations of the initial efforts and, at the Federal Reserve Bank of New York, for being complicit in failing to counter abuses that gave rise to the credit crisis.

On Tuesday, he acknowledged the criticism. "Policy was always behind the curve, always chasing the escalating crisis."