The ACLU in a statement urged the U.S. Congress to "vote to hold White House officials in contempt for refusing to cooperate with legitimate congressional subpoenas."

The ACLU statement said the issue had become "a constitutional crisis that threatens to destroy the separation of powers."

"Presidents have tried in the past to overreach in claiming executive privilege," said Caroline Fredrickson, director of the ACLU Washington Legislative Office. "However, Congress has long served as a check to such abuses of power, slapping the president's hand when needed and pursuing contempt or enforcement actions that eventually resulted in the release of crucial information. Today's Congress must do the same if it wishes to remain a meaningful and independent branch of government."

The ACLU said it "rejected claims that Congress' responsibility to conduct oversight or investigate executive misconduct was somehow less important than its legislative function and therefore not worthy of compulsory enforcement."

"It's do-or-die time for the separation of powers," Fredrickson said. "Congress is facing a historic moment when it can fight for its rightful place in our Constitution or accept the president's continued and sweeping claims of supremacy."

The ACLU noted that U.S. courts "have long supported Congress' authority not only to pass laws, but also to investigate their application. The courts have asserted that claims of executive privilege are a potentially dangerous proposition that should only be applied, and can only be upheld, under narrow circumstances."

The confrontation between the Democratic-controlled 110th Congress and the Bush administration on warrantless surveillance has been escalating in recent weeks, with both sides hardening their positions.