The latest move in the separation of powers fight between the legislative and executive branches came as members of Congress began returning from their Fourth of July recess. An atmosphere of high tension accompanied the resumption of work as a fight also loomed there between majority Democrats and some key Republicans and Bush over his Iraq war policy.

In his letter regarding subpoenas the Judiciary panels issued, Fielding said, "The president feels compelled to assert executive privilege with respect to the testimony sought from Sara M. Taylor and Harriet E. Miers."

"You may be assured that the president's assertion here comports with prior practices in similar contexts, and that it has been appropriately documented," the letter said.

Fielding was responding to a 10 a.m. EDT deadline set by the Democratic chairmen, Sen. Patrick Leahy of Vermont and Rep. John Conyers of Michigan, for the White House to explain it's privilege claim, prove that the president personally invoked it and provide logs of which documents were being withheld.

As expected, Fielding refused to comply. He said he was acting at Bush's direction, and he complained that the committees had decided to enforce the subpoenas whether or not the White House complied.

"The committees have already prejudged the question, regardless of the production of any privilege log," Fielding wrote. "In such circumstances, we will not be undertaking such a project, even as a further accommodation."

Conyers' response left little doubt where the matter was headed.

"Contrary what the White House may believe, it is the Congress and the courts that will decide whether an invocation of Executive Privilege is valid, not the White House unilaterally," the House chairman said in a statement.

The privilege claim on testimony by former aides won't necessarily prevent them from testifying this week, as scheduled.

Leahy said that Taylor, Bush's former political director, may testify as scheduled before the Senate panel on Wednesday. The House Judiciary Committee scheduled Miers' testimony for Thursday, but it was unclear whether she would appear, according to congressional aides speaking on condition of anonymity because negotiations were under way.

The exchange Monday was the latest step in a slow-motion legal waltz between the White House and lawmakers toward eventual contempt-of-Congress citations. If neither side yielded in that circumstance, it would go to a federal court.

The probe into the U.S. attorney firings was only one of several Democratic-led investigations of the White House and its use of executive power spanning the war in Iraq, Bush's secretive wiretapping program and his commutation last week of I. Lewis "Scooter" Libby's prison sentence.

Meanwhile, several Democratic-run investigations are playing out this week as they head toward contempt of Congress citations and, if neither side yields, federal court.

On Iraq, Democrats expect to resume legislative challenges to Bush's policy on the war as the Senate this week takes up a major defense spending bill. The administration has been concerned about an escalation of Iraqi war fervor. So much so that Defense Secretary Robert Gates canceled a four-nation South American tour this week to work with the White House on Iraq policy. unclear whether she will appear.

Sen. Susan Collins of Maine, ranking Republican on the Homeland Security Committee, said Monday there had been "a steady erosion for the president's policy" in Congress because of the "tremendous loss of life among our troops" in June and "the failure of the Iraqi government to pursue the political reforms that are necessary to quell the sectarian violence."

Collins is among the Senate Republicans seeking to see U.S. troops departing Iraq by early 2008. Bush's strategy for a short-term troop increase to stabilize Baghdad and certain parts of Iraq has not been successful, she said.

"The president argued that we needed to undertake the surge in order to give the Iraq government the time, the space to pursue the political reforms," Collins said on CNN. "That hasn't happened. Instead it has been our troops who are making the sacrifice, who are bearing the burden, and that's why you see a real change in support for the Iraq strategy."

In Baghdad Monday, Iraqi Foreign Minister Hoshyar Zebari warned that a quick American troop withdrawal could lead to civil war and the collapse of the Iraqi state.