WASHINGTON (AP) – President Bush, moving toward a constitutional showdown with Congress, asserted executive privilege Thursday and rejected lawmakers’ demands for documents that could shed light on the firings of federal prosecutors.

Mr. Bush’s attorney told Congress the White House would not turn over subpoenaed documents for former presidential counsel Harriet Miers and former political director Sara Taylor. Congressional panels want the documents for their investigations of Attorney General Gonzales’ stewardship of the Justice Department, including complaints of undue political influence.

The Democratic chairmen of the two committees seeking the documents accused Bush of stonewalling and disdain for the law, and said they would press forward with enforcing the subpoenas.

“With respect, it is with much regret that we are forced down this unfortunate path which we sought to avoid by finding grounds for mutual accommodation,” White House counsel Fred Fielding said in a letter to the chairmen of the Senate and House Judiciary Committees. “We had hoped this matter could conclude with your committees receiving information in lieu of having to invoke executive privilege. Instead, we are at this conclusion.”

Thursday was the deadline for surrendering the documents. The White House also made clear that Ms. Miers and Ms. Taylor would not testify next month, as directed by the subpoenas, which were issued June 13. The stalemate could end up with House and Senate contempt citations and a battle in federal court over separation of powers.

“Increasingly, the president and vice president feel they are above the law,” said Senator Leahy, Democrat of Vermont and the Chairman of the Senate Judicary Committee. He portrayed the president’s actions as “Nixonian stonewalling.”

His House counterpart, Judiciary Chairman John Conyers, Democrat of Michigan, said Mr. Bush’s assertion of executive privilege was “unprecedented in its breadth and scope” and displayed “an appalling disregard for the right of the people to know what is going on in their government.”

In his letter, Mr. Fielding said Mr. Bush had “attempted to chart a course of cooperation” by releasing more than 8,500 pages of documents and sending Gonzales and other senior officials to testify before Congress. The White House also had offered a compromise in which Ms. Miers, Ms. Taylor, White House political strategist Karl Rove and their deputies would be interviewed by Judiciary Committee aides in closed-door sessions, without transcripts.

Messrs. Leahy and Conyers rejected that offer. Senator Hatch, Republican of Utah, a member of the Judiciary Committee, said the Democrats should have accepted it.

“We would be much farther ahead in finding out whether there’s any real impropriety here or not,” said Mr. Hatch, a former chairman of the committee. He also said presidents have legitimate reasons to protect the confidentiality of the advice they get.

In his letter, Mr. Fielding explained Mr. 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 advisors and between those advisors 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, Mr. Fielding argued. And he questioned whether the documents and testimony the committees seeking are critically important to their investigations.

It was the second time in his administration that Mr. Bush has exerted executive privilege, said White House deputy press secretary Tony Fratto. The first instance was in December, 2001, to rebuff Congress’ demands for Clinton administration documents.

Tensions between the administration and the Democratic-run Congress have been building for months as the House and Senate Judiciary panels have sought to probe the firings of eight federal prosecutors and the administration’s program of warrantless eavesdropping. The investigations are part of the Democrats’ efforts to hold the administration to account for the way it has conducted the war on terrorism since the Sept. 11, 2001, attacks.

Democrats say the firings of the prosecutors over the winter was an example of improper political influence. The White House says U.S. attorneys are political appointees who can be hired and fired for almost any reason.

Democrats and even some key Republicans have said that Gonzales should resign over the U.S. attorney dismissals, but he has steadfastly held his ground and Bush has backed him.

Just Wednesday, the Senate Judiciary Committee subpoenaed the White House and Vice President Cheney’s office, demanding documents pertaining to terrorism-era warrant-free eavesdropping. “It’s an outrageous request,” White House press secretary Tony Snow said.

“It’s pretty clear that again members of Congress are engaged in an attempt … to try to do what they can to make life more difficult for the White House,” Mr. Snow said. “It also explains why this is the least popular Congress in decades, because you do have what appears to be a strategy of destruction rather than cooperation.”

Separately, the Senate Judiciary Committee also is summoning Gonzales to discuss the program and an array of other matters – including the prosecutor firings – that have cost a half-dozen top Justice Department officials their jobs.

The Judiciary panels also subpoenaed the National Security Council. Leahy said that, like Conyers, he would consider pursuing contempt citations against those who refuse.

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Associated Press Writer Deb Riechmann contributed to this story.