President Bush on Monday rejected subpoenas for two former White House aides and invoked executive privilege, increasing the likelihood of a constitutional showdown.

The White House also refused to turn over documents to the congressional investigation into the firings of several U.S. attorneys last year. Officials said the president would make available his aides to testify in private, off-the-record meetings with committee members and their staffs.

"The president feels compelled to assert executive privilege with respect to the testimony sought from Sara M. Taylor and Harriet E. Miers," White House counsel Fred Fielding wrote in a letter to the chairmen of the House and Senate Judiciary committees.

"The assertion of executive privilege here is intended to protect a fundamental interest of the presidency: the necessity that a president receive candid advice from his advisers and that those advisers be able to communicate freely and openly with the president, with each other and with others inside and outside the executive branch," Fielding wrote.

Senate Judiciary Committee Chairman Patrick Leahy was expected to respond to the invocation in an afternoon speech on the Senate floor. Even before the White House action, he quickly announced his displeasure with the latest message from the White House.

"This latest stonewalling attempt raises troubling questions about what the White House is trying to hide by refusing to turn over evidence it was willing to provide months ago as long as the information was shared in secret with no opportunity for Congress to pursue the matter further," Leahy, D-Vt., said in a statement released from his office on Friday.

"This White House continues to say it is open to cooperation, yet fails to live up to its promises time and again," he continued.

In his response, House Judiciary Committee Chairman John Conyers hinted at the growing likelihood of a legal battle.

"Contrary to 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," he said.

The latest volley in the fight over the attorney firings comes as Congress is about to take up the contentious debate over the war in Iraq as well as growing investigations into the White House's warrantless wiretapping of suspected Al Qaeda members and sympathizers in the U.S.

Miers, who preceded Fielding in his post, and Taylor, a former aide to presidential adviser Karl Rove, are scheduled to testify this week, and may still do so, according to congressional aides speaking on condition of anonymity because negotiations were under way.

Last month, the House and Senate Judiciary committees issued subpoenas for documents and testimony relating to White House involvement in the Justice Department process leading to the firings of eight U.S. prosecutors.

On June 28, Fielding sent a letter to the panels asserting executive privilege and saying the administration would not provide documents and testimony from Miers or Taylor.

The next day, the committee chairmen sent the White House an angry letter asking for a full explanation of its claim of executive privilege, along with documentation. The letter set Monday as the deadline for its request.

Leahy and Conyers said in their June 29 letter that whether or not the White House met the deadline, "we will take the necessary steps to rule on your privilege claims and appropriately enforce our subpoenas backed by the full force of law."

Monday's response from Fielding reasserted the White House's claim to executive privilege without requiring further explanation. He also chastised the congressional leaders' methods of handling the matter.

Fielding once again offered its conciliation: If Congress revokes the subpoenas, the president will provide information previously on the table — including closed-session, off-the-record testimony from Miers and Taylor.

He went on to criticize the tone of the investigation, saying the congressional leaders "prejudged the question" of White House compliance when they wrote the administration's claim "belies any good faith attempt to determine where privilege truly does and does not apply."

"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."

He added: "Although we speak on behalf of different branches of government, and perhaps for that reason cannot help having different perspectives on the matter, it is hoped you will agree ... it is incorrect to say that the president's assertion of executive privilege was performed without 'good faith.'"

Fielding said he was personally relaying the president's "request that further interbranch relations in this matter be distinguished by respect for the constitutional principles of both institutions and marked by a presumption of goodwill on all sides."