WASHINGTON — The chief aide to Attorney General Alberto Gonzales resigned amid reports that President Bush’s White House staff initiated the decision to fire federal prosecutors.

The Justice Department announced the resignation of Kyle Sampson, who was Mr. Gonzales’s chief of staff. Mr. Sampson may be called to testify by Democrats on the Senate Judiciary Committee who are demanding to know more about the firings of eight U.S. attorneys. Democrats have likened the dismissals to a political purge.

“U.S. attorneys have always been above politics, and this administration has blatantly manipulated the U.S. attorney system to serve its political needs,” Senator Schumer of New York said at a news conference in Washington yesterday. Mr. Schumer said Mr. Sampson’s departure “does not take the heat off the attorney general. In fact, it raises the temperature.”

Mr. Sampson is the first high-level official to lose his job over the firings. The House Judiciary Committee yesterday released a number of his e-mails that describe White House involvement in the dismissals.

A Utah native, Mr. Sampson had previously worked with Mr. Gonzales when the attorney general was White House counsel. Before joining the administration, Mr. Sampson was an aide to Senator Hatch, a Republican of Utah.

“Kyle Sampson has served as a key member of my team,” Mr. Gonzales said in a statement. “I am very appreciative of his service, counsel, and friendship during the last six years.”

Mr. Gonzales canceled a trip yesterday to New York and Massachusetts as the administration sought to contain the controversy.

On Capitol Hill, Reps. Nancy Pelosi and Henry Waxman, both Democrats of California, said at a press briefing that they welcomed congressional hearings into the firings. They declined to call for Mr. Gonzales’s resignation, saying they wanted to know more about the matter.

The Judiciary Committee chairman, Senator Leahy, a Democrat of Vermont, said he would summon Mr. Gonzales, Mr. Sampson, and a former White House counsel, Harriet Miers, to testify. Mr. Leahy said he hadn’t decided whether to call presidential adviser Karl Rove, who the White House said yesterday relayed complaints about some prosecutors to Ms. Miers and possibly to Mr. Gonzales.

“I want to find out exactly what happened,” Mr. Leahy said. “We will have hearings and there will be subpoenas, and people will be under oath.”

White House spokesman Tony Snow told reporters traveling with Mr. Bush in Mexico yesterday that Mr. Bush still has confidence in Mr. Gonzales.

Mr. Snow said the White House was considering whether it should “bring in fresh blood” to replace the U.S. attorneys after Mr. Bush was re-elected in 2004. Ms. Miers put together a memo at the “dawn” of Mr. Bush’s second term, suggesting that the administration consider asking all 93 U.S. attorneys to resign, Mr. Snow said.

“That was quickly rejected by the Justice Department,” he said.

“Nobody at the White House was involved in suggesting the firing of 93 attorneys,” Mr. Snow said. “It was not a firm recommendation that you’re firing everybody.” He also said Mr. Bush didn’t make any “recommendation on specific individuals.”

In the e-mails and other documents released yesterday, Mr. Sampson sent Ms. Miers a “Plan for Replacing Certain United States Attorneys” dated November 15, 2006. The plan called for the administration to “simultaneously” call home-state senators about the ousters.

“We’ll stand by for a green light from you,” Mr. Sampson wrote Ms. Miers.

In a January 9, 2006, e-mail response, Mr. Sampson said “wholesale removal of U.S. attorneys would cause significant disruption” of the Justice Department’s work. He recommended that “a limited number” of them “could be targeted for removal and replacement, mitigating the shock to the system” that an “across-the-board firing” would cause.