WASHINGTON — Setting up a constitutional showdown, the White House on Friday asserted executive privilege in denying a request to turn over thousands of pages of documents sought by Congress as part of an investigation involving California's air-quality standards.

"I don't think we've had a situation like this since Richard Nixon was president," said California Democratic Rep. Henry Waxman, chairman of the House Oversight and Government Reform Committee.

Waxman canceled a contempt vote that had been scheduled Friday morning against Environmental Protection Agency Administrator Stephen Johnson and White House official Susan Dudley after the White House informed him of their last-minute decision. The two have refused to turn over subpoenaed documents to Waxman’s panel.

President Bush sought advice from Attorney General Michael Mukasey before making his decision.

"The Office of Legal Counsel is satisfied that the subpoenaed documents fall within the scope of executive privilege," Mukasey said in a letter to Bush, which was released by Republicans on the committee.

Waxman said that Johnson and Dudley wanted to approve a waiver sought by California officials to strengthen the state's air-quality standards, only to have Bush overrule them.

"There are thousands of internal White House documents that would show whether the president and his staff acted lawfully," Waxman said. "But the president has said they must be kept from Congress and the public."

Waxman said the committee will investigate the matter further before deciding how to proceed.

"We will not abandon this matter."

Democratic Rep. Dennis Kucinich of Ohio, a member of the committee, said Bush has demonstrated a contempt of Congress and abused his power. He reiterated his call to impeach the president, saying it's the only way the administration can be held accountable by Congress.

"Can the government be clean if the air is not?" Kucinich asked.