Key White House aides could be made to testify under oath about the firing of federal prosecutors, after a panel in Congress voted to authorise subpoenas.
George W Bush says he does not want confrontation with Democrats
The move could set up a constitutional showdown with the White House, which has vowed to resist such moves.
President George W Bush says he will only allow the aides to speak privately, and not under oath.
Congress wants to question Karl Rove, President Bush's top political adviser, as well as Mr Bush's former lawyer.
Critics say last year's sacking of the eight attorneys was politically motivated.
Mr Bush's Attorney General, Alberto Gonzales, has faced calls to resign over the dismissals.
The president has thus far stood by Mr Gonzales, a long-time confidant from their days in Texas before they came to Washington together.
Mr Gonzales says the prosecutors were dismissed because their performances were below standard.
Congressional investigations have found that Mr Bush's former counsel, Harriet Miers, proposed firing all 93 US attorneys nationwide in 2005.
Mr Gonzales is fighting to hold onto his job
One of the sacked attorneys was replaced by a former aide to Mr Rove.
Mr Bush said on Tuesday that Congress should accept his offer to let his aides testify privately, without oath or transcript.
He vowed to resist any order, or subpoena, for them to testify in public, saying presidential aides would be compromised if they feared having to justify themselves publicly.
"We will not go along with a partisan fishing expedition aimed at honourable public servants," Mr Bush said.
On Wednesday White House counsel Dan Bartlett told National Public Radio that any subpoena would be refused by the administration.
He added such a move would lead the White House to withdraw its offer of private testimony.
The BBC's Jonathan Beale in Washington says the House demand for testimony under oath sets up a constitutional battle between the president and Congress which could end up in the Supreme Court.
The criticism of Mr Gonzales began with the Democrats who now control Congress, but some Republicans have joined the chorus.
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
The Senate voted overwhelmingly - and with bipartisan support - on Tuesday to strip Mr Gonzales of the power to appoint US attorneys without its consent.
And the vote on Wednesday authorising the use of subpoenas to compel White House officials to testify passed on a voice vote with no dissent.
Critics of the prosecutor firings - including some of the prosecutors themselves - say they were removed for investigating Republican officials or failing to investigate alleged vote fraud in support of Democrats.