Libertarians and Democrats say that President Bush’s warrantless surveillance program is a clear violation of the Foreign Intelligence Surveillance Act. Bush and some impressive intelligence professionals say that the program is a vital early-warning defense against Qaeda terrorists and thus an exercise of the president’s inherent constitutional power to defend the nation; this power, they say, trumps any conflicting provisions of FISA.

Both sides are right. But both are wrong to spend their time bickering over the legality or illegality of what Bush has authorized the National Security Agency to do. They should be focusing on how to amend FISA to provide independent checks on abuse of the NSA program without compromising its effectiveness in preventing mass murder.

"FISA is outdated and inadequate and urgently needs reform to make it effective against the jihadist threat," says Philip Bobbitt, a Texas law professor and defense expert who held a high position in President Clinton’s National Security Council and is writing a book on the war against terror.

"The critics are insisting on rules that fly in the face of the strategic realities," Bobbitt adds. "But when the president secretly decides a measure is unconstitutional, and neglects to say so (much less why), he sacrifices the legitimacy that comes from public understanding of his decision and undermines our system of public consent."

Bobbitt is exactly right, on all counts. As to Bush, the man never consults Congress when he can make a grandiose claim of executive power instead. He sometimes seems to have forgotten that the goal is "fighting terrorism, not avoiding oversight," as my colleague Jonathan Rauch detailed in the January 7 issue of National Journal.

Bush should have sought congressional authorization for his NSA program long ago. His excuse for not doing so — even now, he suggests — is that the legislative process would tip off terrorists on how to hide from the NSA. This is utterly unpersuasive and rather alarming. Carried to its logical conclusion, it would argue for ending all congressional oversight and censoring media coverage of all sensitive intelligence and defense activities.

To back up a bit: What is this NSA program, anyway?

The NSA is part of the Defense Department. It is also our largest and most important intelligence agency. Its main job for decades has been to vacuum up billions of phone conversations, e-mails, and other electronic communications abroad: Soviet generals microwaving instructions to their missile silos, Egyptian jihadists phoning their sons in Europe (or Cairo), Osama bin Laden calling Saudi Arabia from Pakistan. All this is uncontroversial, at least in this country.

What’s controversial is the surveillance of communications between people abroad and in America. FISA, a 1978 law of great complexity, forbids such surveillance unless the government (usually the FBI) obtains a warrant from a special federal court based on "probable cause" to believe that the surveillance target is an agent of a foreign power or group (or a criminal).

After 9/11 had dramatized the apocalyptic intent and terrifying capabilities of the global Qaeda-led conspiracy to mass murder Americans, the NSA and the White House reacted as they should have. They saw an urgent need to use the NSA’s vast technological resources for a program of surveillance of communications between suspected Qaeda agents and other people (collaborators and innocents alike) when one party to the communication is in this country and the other is abroad.

"This is hot pursuit of communications entering or leaving America involving someone we believe is associated with Al Qaeda," Gen. Michael V. Hayden, a veteran intelligence professional who had headed the NSA until Bush made him deputy director of national intelligence last year, explained in a compelling January 23 speech.

But FISA stood in the way. So Bush bypassed its cumbersome administrative and judicial review requirements because — notwithstanding narrow exceptions for emergencies — they sometimes make it impossible to move as fast as the terrorists. Bush also bypassed FISA’s "probable cause" burden of proof. As he should have.

If the feds want to spy on a man based on evidence suggesting a 25 percent probability that he is the next Mohamed Atta, they should be able to spy on him, even though most judges would find probable cause to be lacking.

For this reason, I proposed in 2003 that Congress "lower the FISA burden of proof from ‘probable cause’ to ‘reasonable suspicion.’ " It turns out that Bush had already done this in 2001 — secretly and unilaterally. He should now ask Congress to remove doubts about the program’s legality by amending FISA.

The administration says that the NSA program is already legal. It points to the September 18, 2001, congressional vote authorizing use of military force against all those involved in the 9/11 attacks. And it argues that this broadly worded resolution effectively amended FISA by implicitly authorizing Bush to order electronic eavesdropping targeted on Al Qaeda without regard to FISA’s requirements.

Most experts correctly dismiss this argument as extremely weak, for reasons too complex to detail here.

The administration’s fallback argument is stronger, albeit more risky politically: FISA is unconstitutional if interpreted as barring the commander-in-chief from doing what Bush did in September 2001. Back then, it may well have been essential to launch the NSA program without awaiting congressional action.

But the inherent-power claim has weakened with each passing day since then. It is one thing to say that the president has inherent power to disregard an outdated law during an emergency in which immediate action may be the best hope of saving many lives. It is something else to say that the president can secretly continue to disregard that law for over four years without ever seeking to amend it. By doing this, Bush has undermined both his legal and his political positions while inviting a backlash that could jeopardize the NSA program and hurt the presidency. (The peskily pre-emptive Jonathan Rauch made that point, too, in his January 7 column.)

Many critics also argue that even if approved by Congress, the NSA program would violate the Fourth Amendment. I disagree.

It’s true that the courts have generally held most criminal-investigative searches and wiretaps to be unconstitutional absent judicial warrants based on probable cause that evidence of crime will be found. But the Fourth Amendment’s plain text requires only that searches be "reasonable."

It should be obvious that a wiretap designed to prevent a mass murder, when every moment’s delay could make a difference, can reasonably be authorized without requiring as much evidence as a wiretap designed to find burglar tools or a bag of marijuana.

The Supreme Court has dispensed with warrants and probable cause for many types of searches. Examples include warrantless "hot pursuit" of criminal suspects; warrantless car stops and "stop-and-frisk" encounters with pedestrians based on reasonable suspicion (not probable cause) of criminal activity; highway sobriety checkpoints; and mandatory drug testing of transportation workers and high school athletes with no individualized suspicion at all.

The Court has reasoned that "special needs" other than criminal prosecution justify such searches. The quintessential special need is preventing harm to public safety. That is the real (if sometimes unacknowledged) reason why nobody questions the practice of searching all airline passengers. And that is why FISA itself watered down the usual criminal-investigation requirement 28 years ago, by requiring only probable cause to believe that the target is a foreign agent (even absent evidence of crime).

In 2001, as Hayden explained, "the standard by which we decided … what was reasonable would understandably change, I think, as smoke billowed from two American cities and a Pennsylvania farm field."

Hayden is a serious man. The critics who trash every Bush move to fight terrorism without proposing effective alternatives are not. We are at war. And in this era of doomsday weapons, we are in grave danger. The response of many civil-liberties activists and some politicians has been to fight tooth and nail against even the most effective and innocuous new investigative powers. This stance is worse than useless.

Those who call Bush a criminal especially miss the point. After doing the right thing in the wake of 9/11, he has damaged the nation (and arguably broken the law) by unnecessarily prolonging his unilateral defiance of FISA. The president needs fewer yes-men and yes-women and more advisers in the mold of Talleyrand. That was the guy who memorably reproached Napoleon for a more lethal excess of executive power two centuries ago: "Sire, it was worse than a crime. It was a mistake."