US Attorney General Alberto Gonzales has been quizzed by the Senate Judiciary Committee over the sacking of eight US federal prosecutors.
Mr Gonzales has come under pressure to resign
Members of the panel - including many Republicans - challenged the credibility of Mr Gonzales' statements regarding the prosecutors' dismissal.
One Republican senator on the committee called for his resignation.
Mr Gonzales said that while the process leading to the dismissals was flawed, he would make the same decision again.
He said the process by which the prosecutors were sacked was "nowhere near as rigorous or structured as it should have been" but he firmly believed that "nothing improper occurred".
Republican Sen Tom Coburn pressured Mr Gonzales to step down during the panel hearing, saying: "The best way to put this behind us is your resignation."
But Mr Gonzales said his departure would not end the controversy over the sackings and he believed he could still be effective in his role.
The panel's senior Republican, Sen Arlen Specter, also pressed Mr Gonzales, saying his version of events was "significantly if not totally at variance with the facts".
But the White House has reiterated its support for Mr Gonzales - a close ally of President George W Bush - with a spokesperson saying the beleaguered attorney-general still had Mr Bush's "full confidence".
Mr Gonzales is a long-time confidant of Mr Bush from their days in Texas before they came to Washington together.
The BBC's Vanessa Heaney in Washington said Thursday's testimony was seen as the last chance for Mr Gonzales to save his job.
The panel's Democratic chairman, Sen Patrick Leahy, said the Justice Department was "experiencing a crisis of leadership perhaps unrivalled during its 137-year history".
"There's a growing scandal swirling around the dismissal" of the prosecutors, he said.
This is a huge embarrassment for Mr Bush, our correspondent says, with yet another of his allies under the spotlight.
Serve at the discretion of the president, with the approval of the Senate
Prosecute criminal cases brought by the government
Prosecute or defend civil cases in which the government is a party
Collect debts owed to the government
Source: US Department of Justice
And while Congress cannot bring Mr Gonzales down directly, it can create an atmosphere that makes it impossible for him to continue in the role.
The Democratic Party, which now controls Congress, is pushing to expand an investigation into the firings.
Mr Gonzales has repeatedly said the prosecutors were fired because of their job performance and that politics played no role.
But critics say last year's dismissals were meant to halt investigations into Republican officials or punish the attorneys for failing to prosecute Democrats.
Mr Gonzales appeared before the committee three weeks after his former chief of staff, Kyle Sampson, testified that the attorney general was more involved in the firings than he had acknowledged.
There are 93 federal prosecutors in the US who investigate and prosecute court cases for the government.
They can be dismissed at any time and many are often replaced when a new president takes office.