Some of the dismissed prosecutors were involved in corruption investigations and Democrats have questioned whether they were replaced for political purposes - either because they were soft on Democrats or pursuing Republicans too aggressively.

In prepared testimony for the Senate Judiciary Committee, obtained by The Salt Lake Tribune, Sampson argues that U.S. attorneys who perform well are judged on their management and relationship with local law enforcement, but also "their support for the priorities of the president and the attorney general."

"Thus, the distinction between 'political' and 'performance-related' reasons for removing a United States attorney is, in my view, largely artificial," the Cedar City native says in his testimony.

Sampson says "a number of senior Justice Department officials" provided input on which prosecutors should be replaced, and the White House reviewed the list of targeted attorneys, as well.

Sampson doesn't name names and makes no reference to the role of Gonzales or any White House officials, but Democratic senators have made clear they plan to pursue those issues in questioning today.

Adding to the questions Sampson is likely to receive, the Justice Department on Wednesday notified Congress that it provided inaccurate information in a February letter to senators regarding the decision to pick Tim Griffin, a longtime aide to President Bush's political adviser Karl Rove, as U.S. attorney for Arkansas.

Sampson spearheaded the drafting of the letter, which said Rove was not involved in the decision, and cleared it with the White House. The letter contradicts an earlier e-mail in which Sampson acknowledged the Griffin appointment was important to Rove and then-White House Counsel Harriet Miers.

"We sincerely regret any inaccuracy," Richard Hertling, acting assistant attorney general, wrote to Congress members Wednesday.

Sampson is due to appear voluntarily today before the Senate Judiciary Committee, becoming the first official to testify under oath since e-mails raised serious questions about the plan to fire the prosecutors and the testimony Justice Department officials provided to Congress.

Sampson's testimony reaffirms the genesis of the firings that was laid out in thousands of pages of e-mails that have been released: that the White House inquired about replacing all 93 U.S. attorneys after the 2004 election, but Sampson and others believed that more limited changes would be appropriate.

He took the lead on the issue, consulting with other senior officials at the Justice Department and the White House, Sampson says.

"I provided that information to the White House when requested and reviewed it and circulated it to others at the Department of Justice for comment," Sampson says in his testimony.

He says the criteria used to identify which prosecutors should be replaced was not scientific, but it was also not arbitrary. He said he believes the firings were proper, but he failed to prevent congressional inquiries to the firings from becoming "an ugly, undignified spectacle."

"I want to apologize to my former DOJ colleagues, especially the U.S. attorneys who were asked to resign," he says. "This should not have happened. The U.S. attorneys who were replaced are good people, each served our country honorably, and I was privileged to serve at the Justice Department with them."

Because of his failure to organize an appropriate response, Sampson resigned as Gonzales' chief of staff on March 12. Gonzales said Sampson failed to fully share details of the plan to replace U.S. attorneys with department officials testifying before Congress.

"I failed to organize a more effective response to questions about the replacement process, but I never sought to conceal or withhold any material fact about this matter from anyone," Sampson says.