A bipartisan congressional consensus seems to be emerging: First, the Bush administration’s eavesdropping program (or something like it) should be continued to try to avert Al Qaeda attacks. Second, such spying should be subjected to oversight by Congress and the courts, regardless of whether President Bush wants it to be.

What kind of oversight? And how should Congress update the Foreign Intelligence Surveillance Act of 1978 to deal with the unprecedented magnitude of the internal security threat posed by jihadists who covet doomsday weapons? If we get the answers wrong, we will end up with weaker defenses against terrorism, insufficient protections against the abuse of civil liberties, or both.

The most specific proposal so far is terrible — Senate Judiciary Committee Chairman Arlen Specter’s notion that Congress should ask the secret Foreign Intelligence Surveillance Court to weigh in on whether the program is constitutional. That would amount to a request for an advisory opinion on a broad matter of policy. Courts have spurned such requests since the dawn of the Republic. The reason is that their constitutional mandate is to resolve individual cases and controversies — not to help set broad policies, let alone to micromanage presidential spying on the enemy in wartime.

Specter aside, we need to adapt the judicial role in enforcing the Fourth Amendment ban on "unreasonable searches and seizures" to current realities. The courts have traditionally decided in advance (or, when speed is crucial, after the fact) whether the government has "probable cause" to target an individual for searching or electronic eavesdropping. But as Judge Richard Posner of the U.S. Court of Appeals for the 7th Circuit noted this Wednesday in a Wall Street Journal op-ed, this model is "hopeless as a framework for detecting terrorists," which amounts to "a search for the needle in a haystack."

One of the best hopes for finding jihadist infiltrators is to scan the communications of many thousands of innocent people — based on clues and patterns suggesting that a higher percentage of them than of the general population are likely to be terrorists or in touch with terrorists. Thanks to computer technology, the government can do this without violating the Fourth Amendment.

To illustrate, consider my educated guess at how Bush’s National Security Agency program sifts through calls and e-mails between the United States and foreign countries for evidence of Qaeda conspiracies.

Since long before 9/11, NSA computers have apparently used sophisticated algorithms to screen communications by the millions and to flag thousands that contain key words or fit certain patterns suggesting some rather remote possibility of a connection to terrorism. Absent scrutiny by human eyes or ears, this initial filter should not implicate either FISA or the "reasonable expectations of privacy" the Supreme Court has held to be protected.

A second filter apparently involves at least cursory inspection by NSA personnel of the contents of these thousands of computer-flagged communications, in order to identify the relatively small fraction that seem worthy of a closer look. As FISA is written, even cursory inspection requires a judicial warrant based on having "probable cause" to believe that the sender or the recipient is a foreign agent. But at this stage of the filtering process, the government has no evidence that comes close to "probable cause." (This may be one reason that Bush authorized the NSA to bypass FISA’s judicial-review requirements.)

FISA should be amended to exempt this second NSA filter from judicial oversight if the process is structured to avoid violating anyone’s reasonable expectation of privacy. This can easily be done. No one has a reasonable expectation of privacy in communications that the government cannot connect to him or her. So the key is to design the second filter — and the software — to shield the identity of the sender and receiver from the government employees scanning the communications. (Perhaps NSA is already using such a shield.)

The second filter leaves the NSA with a few hundred or a few thousand communications suspicious enough to warrant close scrutiny, including names. But even at this stage, the government might have difficulty showing that it has "probable cause" to consider the parties Qaeda operatives.

The definitions of "probable cause" in case law are so vague and elastic that they could arguably be stretched to encompass, say, a governmental guess that one in 10 wiretap targets will turn out to be a terrorist. But given the uncertainty about how courts would rule, the best solution might be to amend FISA to give the government two options.

The first would be to try to persuade the FISA court to find probable cause and issue a warrant. The second would be to proceed without court approval if a high-level official certifies in writing that reasonable grounds exist to think that the likelihood that an individual wiretap will intercept Qaeda communications outweighs the intrusion into the target’s privacy. Officials could take into consideration factors such as the tap’s expected duration.

In amending FISA, how could Congress guard against abuse or overuse of this power to eavesdrop without advance judicial approval? First, Congress should bar evidence obtained that way from being used for any prosecution that does not involve international terrorist activities. Second, Congress should tell the courts that — if necessary to avoid striking down the entire provision — they should bar use of such evidence even in terrorism prosecutions absent any (after-the-fact) judicial finding that the government did indeed have reasonable grounds for the eavesdropping. Third, Congress should bar putting any such surveillance target on any list of "suspected terrorists" absent a certification to the FISA court by a high-level official that the surveillance evidence establishes probable cause to believe that he is a terrorist.

Fourth, Congress should require tighter oversight by its own leadership or, even better, by an elite commission of security-cleared officials and private citizens that would report to Congress at regular intervals. And fifth, Congress should strengthen the formidable array of penalties for misuse of information obtained through such surveillance — penalties that, had they existed, might have deterred FBI Director J. Edgar Hoover from misusing wiretaps of Martin Luther King Jr. and many others.

The Supreme Court should and probably would uphold such a law as consistent with the Fourth Amendment. The Court’s precedents recognize the need for exceptions to the general rule that searches be based on warrants and probable cause. And no better reason for an exception exists than preventing terrorist attacks.

Civil libertarians, understandably alarmed by Bush’s push to aggrandize his powers, are demanding judicial oversight of all electronic surveillance. But they are overreacting.

One telltale libertarian conceit is that courts should be empowered to sharply curb electronic surveillance in order to force the executive branch to shift counterterrorism resources to (supposedly) more effective activities, such as recruiting additional undercover informers. But judges don’t have the information, expertise, staff, or constitutional authority to second-guess such strategic choices.

Nor are judges immune from error, or from the temptation to usurp the powers of elected officials. The federal district judges who sit part-time on the 11-member FISA court are fine public servants. But they have done some dumb things.

One was their insistence on maintaining the now-infamous "wall" between foreign intelligence and law enforcement even after it had contributed to our vulnerability to the 9/11 attacks, and even after Congress sought to tear it down by adopting the USA PATRIOT Act. In May 2002, a FISA court ruling implied that the government could not get a warrant if its primary purpose was to prosecute the target for international terrorism or espionage. I say "implied" because a glaring internal inconsistency left the meaning of the 7-0 decision unclear.

Fortunately, the three-judge FISA Court of Review reversed that decision in November 2002, holding it contrary to FISA’s stated purpose of "protecting against" terrorists and spies. One good way to protect against them is to prosecute and imprison them.

The review court also noted the possibility that the lower court’s confusing rules "may well have contributed, whether correctly understood or not, to the FBI missing opportunities to anticipate" the 9/11 attacks. In particular, FBI supervisors had construed these rules as barring a criminal-investigative agent from hunting for Khalid Almidhar, a Qaeda operative who, the CIA had belatedly told the FBI, was in the United States. The frustrated FBI agent warned in a memo, "Someday, someone will die" because of the FISA court’s wall. Thirteen days later, Almidhar helped crash an airliner into the Pentagon.

Courts do have a role to play in protecting individual rights. But giving the judiciary too much power to second-guess the executive branch could be a cure worse than the disease.