A former White House aide has said that President George W Bush was not involved in the controversial firing of eight federal prosecutors in 2006.
Sara Taylor said she never spoke to Mr Bush about the matter
But testifying before the Senate Judiciary Committee, Sara Taylor refused to answer certain questions.
She invoked President's Bush powers to overrule congressional subpoenas for her - and another former White House aide - to testify on the record.
The row hinges on whether the attorneys were sacked for political motives.
Democrats in Congress have been trying to force the resignation of Attorney General Alberto Gonzales, accusing him of firing the prosecutors for political reasons and then lying about the reason for their dismissal.
Some Republicans have also backed the moves.
But the White House maintains the prosecutors were dismissed because they were ineffective.
"I don't believe that anybody did anything wrong or improper with respect to this issue," Ms Taylor, a former White House political director, told the committee.
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
She said she had never spoken to Mr Bush about replacing the prosecutors or attended a meeting with the president about the matter.
But she declined to answer a number of questions, including who decided which US attorneys to sack and why.
Ms Taylor cited a letter from White House counsel Fred Fielding that advised her not to answer questions "concerning White House considerations, deliberations or communications".
The BBC's Vanessa Heaney in Washington says Ms Taylor was in an impossible position - to remain loyal to her former boss, or answer the tough questions from the senators. She chose the former.
On Monday, President Bush invoked executive privilege to deny requests by Congress for the testimony of Ms Taylor and Harriet Miers, another former aide.
Ms Miers has been summoned to testify before the committee on Thursday.
Democratic leaders say they could go to court to challenge Mr Bush's move.
He invoked the same little-used power last month to withhold subpoenaed documents.
The White House says Mr Bush is acting in good faith and has offered to let the aides do off-the-record interviews.