US Attorney General Alberto Gonzales has faced tough questioning from a Senate judiciary panel on the sacking of eight federal prosecutors.
Mr Gonzales was also questioned on the White House's use of wire-tapping for terrorism surveillance, which critics have said is illegal.
"I don't trust you," said the panel's head, Democratic Senator Patrick Leahy.
President George W Bush has resisted repeated calls to sack Mr Gonzales, who denies any wrongdoing.
"With a history of civil liberty abuses and cover-ups, this administration has squandered our trust," said Senator Leahy.
"I am not willing to accept a simple statement of 'trust us'," he said. "I don't trust you."
Resignation resisted
Senator Arlen Specter, the committee's senior Republican, suggested that Mr Gonzales appoint a special prosecutor to investigate the sacking of the eight federal prosecutors.
He said such a move might be necessary to ensure congressional oversight of the Bush administration.
"The constitutional authority and responsibility for congressional oversight is gone," he said.
Q&A: Fired prosecutors row
"If that is to happen, the president can run the government as he chooses, answer no questions."
The Senate Judiciary Committee hearing is the latest in a series of hearings looking into last year's dismissals.
Mr Gonzales has been under pressure to resign over the row, which centres on whether the firings were politically motivated.
Mr Bush earlier this month invoked executive privilege to keep former White House counsel Harriet Miers and another former White House aide from testifying.
Backed by Mr Bush, Mr Gonzales has so far resisted all calls - some from senior Republicans - to resign over the dismissal of the prosecutors.
In a statement prepared for the hearing, Mr Gonzales said: "I could walk away, or I could devote my time, effort and energy to fix the problems.
"Since I have never been one to quit, I decided that the best course of action was to remain here and fix the problems."
Wiretapping row
Mr Gonzales also faced questions on Tuesday relating to the Bush administration's wiretapping programme.
US ATTORNEYS
He denied an account that in 2004 he pressured then-Attorney General John Ashcroft to reauthorise Mr Bush's controversial domestic spying programme while he was ill in hospital.
Earlier this year, Mr Ashcroft's deputy James Comey told the committee he had been angered by the appearance of Mr Gonzales and White House chief of staff Andy Card at Mr Ashcroft's bedside.
"I thought I had just witnessed an effort to take advantage of a very sick man who did not have the powers of the attorney general," Mr Comey testified in May.
Speaking about the meeting for the first time, Mr Gonzales said he and Mr Card had been asked by congressional leaders to ensure that the surveillance programme did not expire.
They had gone to inform Mr Ashcroft, he said, but they did not press him when it became apparent he was not competent at that time to reauthorise the programme.
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