The day also was the public debut of Mr. Geithner, the man President Obama has called “the chief economic spokesman for my administration.” And it was a chance for him to begin reintroducing himself after the flap over his failure to pay some federal taxes nearly derailed his confirmation and undercut the credibility he has enjoyed and now needs badly.

Mr. Geithner, 47, becomes the face of the government’s economic rescue efforts after months as the junior partner with his predecessor as Treasury secretary, Henry M. Paulson Jr., and the Federal Reserve chairman, Ben S. Bernanke.

Two weeks into his job, Mr. Geithner has taken full responsibility for issues on which Lawrence H. Summers, his boss at the Treasury in the 1990s and now the chief economic adviser in the White House, had filled in while Mr. Geithner weathered his confirmation controversy.

Mr. Obama gave Mr. Geithner’s speech — in the Treasury Building’s gilded Cash Room on Tuesday — a big advance buildup at his news conference Monday evening. “He’s going to be terrific,” Mr. Obama said, adding, “I’m going to make sure that Tim gets his moment in the sun.”

The Democratic chairman of the Senate Banking Committee, Christopher J. Dodd of Connecticut, introduced Mr. Geithner, making the case that “he understands the enormity” of the financial crisis in a booming voice that contrasted with Mr. Geithner’s initially nervous start.

Mr. Geithner, gaining some confidence, delivered his remarks in simple sentences using a teleprompter, suggesting the importance of communicating a complex issue both to markets and to taxpayers who are reluctantly financing the bailout.

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Mr. Paulson was widely criticized as being inarticulate and faulted for sending mixed signals that occasionally disturbed markets and investors.

In his testimony, Mr. Geithner acknowledged that he risked criticism for not providing more details. He 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.

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Among those was Mr. Paulson’s redefinition of the bailout program after he had sold it to Congress, a shift that unsettled financial markets further. Mr. Geithner did not specify that episode and otherwise has declined to criticize the predecessor with whom he worked. Instead, he has come in for blame as well, both for his role in the miscalculations of the initial efforts and, at the New York Fed, for being complicit in failing to counter abuses that gave rise to the credit crisis.

In his speech, he acknowledged the criticism. “Policy was always behind the curve, always chasing the escalating crisis,” he said, adding that the first effort had proven “inadequate.”

Senator Richard C. Shelby of Alabama, the senior Republican on the Senate Banking Committee, told Mr. Geithner that he and other regulators “failed the American people.”

While Mr. Geithner accepted some blame, he took credit for regulatory efforts at the Fed that “made a substantial difference.”

As Treasury secretary, Mr. Geithner will take a lead role in overhauling the financial regulatory system and he promised “comprehensive reform.”

For the speech that started Mr. Geithner’s day, the guests included Mr. Bernanke, who will be his chief partner under the revised plan, as well as officials from the Treasury and White House, financial regulators, industry lobbyists and representatives of both parties in Congress. Mr. Summers was not present. He was at a White House meeting on the fiscal 2010 budget that will be released later this month.

Though Mr. Obama had said on Monday that Mr. Geithner would be holding a press conference, after the speech at the Treasury, the secretary left the room without taking questions from dozens of reporters seated behind five rows of invited guests. But he immediately went to do live interviews for the CNBC and Bloomberg business news channels. Among the 17 cameras that filmed him was one for a possible “60 Minutes” profile.

The invited included Spencer Bachus of Alabama, the senior Republican on the House Financial Services Committee. He said Mr. Geithner faced “a very long process of winning back credibility” with the public after the tax controversy. But, he added, “opening the drawer and going through the dirty laundry just doesn’t help us at this point. And he is very capable.”