WASHINGTON — Attorney General Alberto Gonzales was briefed regularly over two years on the firings of federal prosecutors, his former top aide said yesterday, disputing Mr. Gonzales’s claims he was aware of the dismissals from afar and newly undercutting his already shaky credibility.

Mr. Gonzales and a former White House counsel, Harriet Miers, made the final decision on whether to fire the U.S. attorneys last year, said Kyle Sampson, the attorney general’s former chief of staff.

“I don’t think the attorney general’s statement that he was not involved in any discussions of U.S. attorney removals was accurate,” Mr. Sampson told a Senate Judiciary Committee inquiry into whether the dismissals were politically motivated.

“I remember discussing with him this process of asking certain U.S. attorneys to resign,” Mr. Sampson said.

Mr. Sampson’s testimony, for the first time, put Mr. Gonzales at the heart of the firings amid ever-changing Justice Department accounts of how they were planned.

Mr. Gonzales has said repeatedly that he was not closely involved in the firings and largely depended on Mr. Sampson to orchestrate them.

Mr. Sampson resigned March 12. A day later, Mr. Gonzales said he “never saw documents. We never had a discussion about where things stood” in the firings.

The White House stepped back from defending Mr. Gonzales even before Mr. Sampson finished testifying.

“I’m going to have to let the attorney general speak for himself,” White House spokeswoman Dana Perino said as Mr. Sampson entered his third hour in front of senators. Ms. Perino made it clear that Mr. Gonzales needs to explain himself to Congress — and quickly.

The attorney general is not scheduled to appear publicly on Capitol Hill until April 17. “I agree three weeks is a long time,” Ms. Perino said.

The Justice Department did not have an immediate comment about Mr. Sampson’s testimony.

A growing number of Democrats and Republicans has called for Mr. Gonzales to step down. Department spokesman Brian Roehrkasse said yesterday the attorney general has no plans to resign.

The Senate committee chairman, Senator Leahy, a Democrat of Vermont, stopped short of calling for Mr. Gonzales’s ouster. But Mr. Leahy reminded a reporter yesterday, “I voted against him,” when the Senate confirmed the Mr. Gonzales as the nation’s top law enforcer in 2005.

“If the president feels Mr. Gonzales is upholding the highest level of professionalism that he wants in his administration and that the president wants to be remembered for, then he’ll stay on,” Mr. Leahy said.

The stony-faced Mr. Sampson, a longtime and loyal aide to Mr. Gonzales, said other senior Justice Department officials helped to plan the firings, which the White House first suggested shortly after President Bush won a second term in 2004.