Washington is immersed in a furious debate over the legality of the National Security Agency's warrantless surveillance program - and the argument's outcome may affect the balance of power in the US government for decades to come.
That is what a bipartisan group of US lawmakers believe, in any case, as they struggle to respond to the White House's assertions of broad powers in the surveillance case.
Unless Congress asserts authority over the program via some form of legislation, some legislators and legal scholars assert, it risks becoming less relevant on important questions of war and national security than it is today.
"This is a defining issue in the constitutional history of the United States," constitutional lawyer Bruce Fein testified Tuesday before the Senate Judiciary Committee.
Since late last year, when news reports revealed the existence of the NSA program, the Bush administration has staunchly defended it as legal. Congress implicitly authorized the program when it voted in 2001 to authorize use of force in the war on terror, claims the White House.
In addition, officials say, the president's inherent authority as commander in chief allows him to take any steps he deems necessary in the defense of the United States.
Some key Republican lawmakers, such as Sens. Lindsey Graham of South Carolina and John Warner of Virginia, Senate Armed Services Committee chairman, have joined Democrats in voicing worry about the extent of power claimed by the executive branch in the eavesdropping issue.
The chairman of the Senate Judiciary panel, Sen. Arlen Specter (R) of Pennsylvania, is leading one of the main efforts to draw up legislation on the issue.
Under a draft version of Senator Specter's bill, the surveillance program would come under the authority of the secret court created by the 1978 Foreign Intelligence Surveillance Act (FISA). The legislation would require the administration to get approval from the court every 45 days for the controversial surveillance program to continue.
Specter is also calling for the FISA court to determine whether the program is constitutional - although a number of legal experts question whether any such ruling would be an inappropriate advisory opinion, or determination of a case without a plaintiff.
While insisting that the program is legal, the White House has indicated that it would work with Congress to codify the law in this area, if necessary. It prefers a proposal from Sen. Mike DeWine (R) of Ohio, which would exempt the program from the FISA law and set up a special congressional committee to provide oversight and review of eavesdropping cases.
The importance of this debate, say experts, lies in the fact that it bears directly on questions of power between the branches that have been debated since members of Congress wore breeches and wigs.
Its outcome will have far-reaching effects, since any shift in this balance tends to persist, say legal scholars. In addition, the White House is in essence asserting privilege in an area that Congress has specifically addressed, via the 1978 FISA statute.
"The president is asserting an inherent constitutional authority in 'wartime' that allows him to ignore the plain meaning of the FISA law," says Thomas Mann, a senior fellow at the Brookings Institution, a Washington think tank. "How it is ultimately resolved will help define the limits of presidential power."
At Tuesday's Senate Judiciary hearing, some experts said it is important to remember the context of the debate. The "war on terror" is not just an empty phrase, they said, but an accurate description of a struggle between the US and enemies that would do it grievous harm.
The US is the battlefield, as much as Afghanistan, former CIA Director James Woolsey told the panel. The NSA must be able to move quickly to eavesdrop on conversations that have a domestic component. "The country has been invaded, albeit not occupied," said Mr. Woolsey.
While expanded congressional oversight of the program may be appropriate, the FISA law was not designed to deal with a fast-moving, internal enemy, he said.
But Harold Hongju Koh, dean of Yale Law School, called the program "blatantly illegal." "Fighting terrorism outside the law is deeply counterproductive," he said.
Any solution will likely be a political agreement, as opposed to judicial-branch rulings, say experts. For one thing, a legal result would take years. The US Supreme Court probably would not rule on these questions until long after the Bush team leaves office.
And the very nature of the executive branch's claims to power in this area means that the White House might ignore legislation with which it does not agree.
"Reasonable minds have been differing on these questions since Madison and Hamilton," Doug Kmiec, a law professor at Pepperdine University, told the Judiciary panel. "It's important not to have recriminations, but to pursue the issue of what is the appropriate course as we go forward."