US President George W Bush has invoked executive privilege to deny requests by Congress for the testimony of two aides over the firing of federal prosecutors.
Harriet Miers is one of the former White House aides subpoenaed
The row hinges on whether the attorneys were sacked for political motives.
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.
The pair in question are Harriet Miers, former White House counsel, and Sara Taylor, former political director for the White House.
Ms Miers has been summoned to appear under oath before the Senate Judiciary Committee on Wednesday and Ms Taylor to testify before the House Judiciary Committee the next day.
The Democratic heads of the two judiciary committees had set a deadline of Monday for the White House to explain the first invocation of privilege last month and to log what documents were being withheld.
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
In a letter to congressional leaders, White House counsel Fred Fielding said their demands were "unreasonable" because they represented "a substantial incursion into presidential prerogatives".
John Conyers, chairman of the House Judiciary Committee, responded with a warning that Congress was prepared to go to court over the matter.
"Contrary to what the White House may believe, it is Congress and the courts that will decide whether an invocation of executive privilege is valid, not the White House unilaterally," he wrote.
The row over the firing of eight federal US attorneys in 2006 has been building for several months.
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 fired because they were ineffective. Mr Bush has dismissed the attempts to remove Mr Gonzales as being themselves political.
US presidents have occasionally used executive privilege to try to prevent themselves and their aides from having to answer questions or give information to Congress or the courts.