But senators, for the most part, seemed unimpressed. Senator Patrick J. Leahy of Vermont, the committee chairman, said the Bush administration had provided “a series of shifting explanations and excuses and lack of accountability or even acknowledgment of the seriousness of this matter.”

And Senator Specter said that while it was still not clear whether United States attorneys like Carol C. Lam in California or David C. Iglesias in New Mexico had been removed with the intention of affecting political-corruption investigations by their offices, the broader impact of the firings had been devastating.

“It is generally acknowledged that the Department of Justice is in a state of disrepair, perhaps even dysfunctional, because of what has happened, with morale low,” Mr. Specter said. “U.S. attorneys across the country do not know when another shoe may drop, whether they may be asked to resign for a bad reason.”

Democrats in particular expressed deep doubt at Mr. Sampson’s argument that politics were not involved in the dismissals. “In the last seven weeks, we’ve learned that Attorney General Gonzales was personally involved in the firing plan, after being told that he wasn’t,” Mr. Schumer said. “We’ve learned that the White House was involved, involved, after being told that it wasn’t; we’ve learned that Karl Rove was involved, after being told that he wasn’t,” the senator said, alluding to the president’s chief political adviser. “And we have learned that political considerations were very important, after being told that they weren’t.”

Mr. Sampson opened with a prepared statement, parts of which had been obtained by the news media late Wednesday.

He argued that the list of United States attorneys to be fired had been compiled with the involvement of “a number of senior Justice Department officials”; that the process, while not “scientific,” was also not “random or arbitrary”; and that the United States attorneys had been appropriately judged by management skills, relationships with other officials and “their support for the priorities of the president and the attorney general.”

“The distinction between political and performance-related reasons for removing a U.S. attorney is, in my view, largely artificial,” Mr. Sampson said.