The US House of Representatives may consider contempt proceedings against a former White House aide after she refused to testify at a hearing.
The panel addressed Harriet Miers's empty chair
Former White House counsel Harriet Miers had been subpoenaed by Congress to speak about the controversial firing of eight federal prosecutors in 2006.
But she instead obeyed an order by President George W Bush not to attend.
Congress could now go to court to challenge Mr Bush's right to invoke executive privilege to shield aides.
On Wednesday, another subpoenaed White House aide appeared before a Senate panel but refused to answer certain questions.
Testifying before the Senate Judiciary Committee, former White House political director Sara Taylor invoked President's Bush powers to overrule congressional subpoenas as the basis for her limited testimony.
Among the questions to which she declined to respond were 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".
'Not legally valid'
The row hinges on whether the US attorneys were sacked for political motives.
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
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.
Addressing Ms Miers's empty chair at Thursday's hearing, Democratic Representative Linda Sanchez ruled that Mr Bush's executive order giving his aides immunity from questioning was "not legally valid".
"Ms Miers is required pursuant to the subpoena to be here now," she said.
The White House had previously offered to let Ms Miers and Ms Taylor give off-the-record interviews to Congress but said it would not allow them to give evidence under oath.
Mr Bush invoked the same little-used executive privilege power last month to withhold subpoenaed documents relating to the firings.
The White House insists Mr Bush is acting in good faith.