Congressional Democrats took the first steps Friday in what could be a long march to court, with the Bush administration and Congress in a legal tug-of-war over executive and legislative branch powers.

In a letter to White House counsel Fred Fielding, the heads of the Judiciary committees demanded an explanation in 10 days of why the White House claimed executive privilege on subpoenaed documents and vowed to invoke "the full force of law."

The fight centers on an investigation that Democrats initially undertook into the firings of several U.S. attorneys, but which has since branched out to scrutiny of the administration's terrorism-era warrantless wiretapping and Attorney General Alberto Gonzales' stewardship of the Justice Department.

"The veil of secrecy you have attempted to pull over the White House by withholding documents and witnesses is unprecedented and damaging to the tradition of open government by and for the people that has been a hallmark of the republic," Rep. John Conyers, D-Mich., and Sen. Patrick Leahy, D-Vt., told Fielding.

They gave the White House until July 9 to furnish the factual and legal bases for the executive privilege claim and documentation that President Bush personally signed off on it.

Whether or not the White House meets 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," Leahy and Conyers wrote.

At issue is Congress' investigation of whether the White House improperly ordered the dismissals of U.S. attorneys — and the committees' demands for internal Bush administration documents. Without an agreement on the subpoenaed documents, the dispute proceeded in slow motion toward contempt citations and, possibly, a constitutional showdown in federal court.

The White House did not immediately respond to the chairmen's tough talk, but Fielding made clear a day earlier that he believes all of the subpoenaed documents are protected by executive privilege.

The very phrase amounts to fighting words.

Throughout the nation's history, presidents have repeatedly asserted executive privilege to keep secrets from the courts, the Congress and most anyone else.

Over the years, Congress and the White House have avoided a full-blown court test about the constitutional balance of power and whether the president can refuse demands from Congress. Lawmakers could vote to cite witnesses for contempt and refer the matter to the local U.S. attorney to bring before a grand jury. Since 1975, 10 senior administration officials have been cited but the disputes were all resolved before getting to court.

Fielding on Thursday explained Bush's position on executive privilege this way: "For the president to perform his constitutional duties, it is imperative that he receive candid and unfettered advice and that free and open discussions and deliberations occur among his advisers and between those advisers and others within and outside the Executive Branch."

This "bedrock presidential prerogative" exists, in part, to protect the president from being compelled to disclose such communications to Congress, Fielding argued.

Democrats - and some Republicans - responded with a rhetorical lashing of the administration's "Nixonian" conduct.

"We urge the president to reconsider this step and withdraw his privilege claim so the American people can learn the truth about these firings," the Conyers and Leahy wrote.

Those facts might have come out had the chairmen accepted Fielding's original offer to allow administration officials to testify in private, without a transcript, the president's lawyer pointed out Thursday. Lawmakers rejected that offer, however, demanding that a record be made of the interviews.

Sen. Arlen Specter, R-Pa., urged a pragmatic response to Fielding's claim of privilege, saying that if the committee accepted the private-interview offer it could always issue subpoenas later.