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.

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.

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.”