Top White House adviser Karl Rove is at the centre of a row over potentially lost e-mails that could shed light on the firing of prosecutors last year.
Whether Karl Rove had a role in the firing of prosecutors is at issue
Democrats probing the dismissals wanted to see the e-mails as part of their inquiry - and expressed disbelief they could have been deleted.
The White House and Republican National Committee admitted they may have lost e-mails from Mr Rove and others.
Attorney General Alberto Gonzales faces calls to resign over the sackings.
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
He is due to testify before US senators about the dismissals - which critics claim may have been politically motivated - on Tuesday.
His appearance is widely seen as a last opportunity to save his job, says the BBC's Vanessa Heaney in Washington.
Mr Gonzales says the eight federal attorneys were fired because of their job performance and that politics played no role.
Congress has called for Mr Rove and other White House aides also to testify in public, under oath, about their role in the process. President George W Bush has rejected the demand.
'Dog ate homework'
In the latest twist of the inquiry, the White House has said that some of its staff, including Mr Rove, wrote some e-mails using Republican National Committee (RNC) accounts.
US attorney general is under pressure over the dismissals
This was done to avoid breaching a law that prevents government employees conducting party political activities using government property.
However, the White House has said that some of the e-mails sent from the RNC accounts were on official business and may have been wrongly deleted from the RNC server.
Some of them may also have related to the dismissal of the eight federal prosecutors, the White House has said.
Democrats have questioned whether the aides used RNC accounts in order to sidestep record-keeping requirements.
Senate Judiciary Committee chairman Patrick Leahy challenged whether the e-mails - which the panel has sub-poenaed - were really missing.
"I don't believe that," he said. "It's not a question of e-mails being lost, it's a question of e-mails you don't want to retrieve."
He dismissed the notion that e-mails could be permanently deleted these days as "like saying the dog ate my homework".
White House spokeswoman Dana Perino insisted on Friday that any mistakes made were not deliberate.
"We've seen no basis to conclude that anyone intentionally or improperly used the RNC e-mail," she said.
Separately, the White House revealed that up to 5m e-mails may have been lost from aides' government accounts during the switch to a new computer system in 2002-2003.