WASHINGTON — Escalating a fight with Democrats on Capitol Hill, the White House on Friday invoked executive privilege in refusing to turn over documents to a congressional committee investigating the Environmental Protection Agency's decision to deny California permission to implement its own vehicle emission standards.

The Bush administration asserted executive privilege hours before the House Oversight and Government Reform Committee was to vote whether to bring contempt of Congress proceedings against EPA administrator Stephen L. Johnson and Susan Dudley, administrator of regulatory affairs in the White House Office of Management and Budget, for refusing to turn over subpoenaed documents.

Committee Chairman Henry A. Waxman, D-Calif., put off a vote on the contempt resolutions while he considers his options.

"I don't think we've had a situation like this since Richard Nixon was president," he said, appearing determined to press ahead, even if it leads to a court fight. "We don't know whether this privilege that's being asserted is valid or not."

Presidents since George Washington have claimed rights to executive branch confidentiality, according to the nonpartisan Congressional Research Service. The Bush White House invoked executive privilege to prevent officials from testifying about the dismissal of nine U.S. attorneys in 2006. President Clinton also cited presidential privilege during investigations pertaining to the Monica Lewinsky scandal and on other issues.

House and Senate committees have been investigating what role the White House played in EPA decisions preventing California and other states from enacting tougher emissions rules than the federal government and in approving new ozone pollution standards.

The administration's claim of executive privilege was the latest twist in the escalating legal and political battle over California's efforts to implement its own law to combat global warming. Critics of the EPA decision contend it was based on politics, not the law and science.

In asserting executive privilege in the EPA probe, the administration made public a copy of a letter sent to the president by Attorney General Michael B. Mukasey that said that releasing internal documents "could inhibit the candor of future deliberations among the president's staff."

EPA spokesman Tim Lyons said the agency has provided the committee with more than 7,000 documents and devoted 2,200 hours of staff time to respond to requests for information and called it "disappointing" that the committee has decided to "politicize environmental regulations."

Jim Nussle, OMB director, took issue with Waxman's "sudden and unwarranted" move to consider contempt proceedings, noting that Dudley had appeared before Waxman's committee last month and was asked "only four questions"—and only one by Waxman.

"There is no valid reason for moving from mutual cooperation to unilateral confrontation," Nussle wrote Waxman.

Waxman said: "I am very disappointed and disturbed that the administration is keeping this information from us, and I think we have a right to it."