WASHINGTON - The two congressional committees that oversee America's intelligence agencies have begun what promise to be riveting hearings into what went wrong before Sept. 11.

There is nothing that Congress likes better than a high-profile investigation into executive branch ineptitude and/or malfeasance. Which is not to say such inquiries are not serious business and hard work; they are both.

But as Congress bores into the growing body of evidence that the CIA, the FBI and other components of the intelligence community dropped the ball, there will be a ghost at the party that it will try hard to ignore: the unacknowledged failure of congressional oversight itself.

Because intelligence and democracy mix about as well as oil and water, most governments, including most democracies, have traditionally given intelligence agencies broad exemption from the rules that apply to other government institutions, including legislative review.

The U.S. approach has been strikingly different. Since World War II, particularly since the 1970s, this country has gradually constructed a system of legislative oversight that centered on two committees, one in the House and the other in the Senate.

By the beginning of the 1980s, a mature mechanism was in place that worked remarkably well - so well that other countries, including Britain and Canada, have tried to replicate it. The Senate Select Committee on Intelligence (SSCI) was a case in point.

From its inception through the 1980s, the SSCI was perhaps the only genuinely nonpartisan (or bipartisan) committee in the Senate. The chairman from the majority party and the vice chairman from the minority acted, in effect, as co-chairmen. An expert staff functioned without reference to partisan affiliation or even identification.

Behind sealed doors, out of sight of the press and public, members conducted their business as colleagues, not as Democrats or Republicans. In return for power over, and access to, the intelligence agencies, Congress effectively agreed to keep secrets secret and conduct its oversight in a nonpartisan, professional manner.

Throughout the 1980s, the system of oversight worked remarkably well because Congress fulfilled its part of the implicit bargain. In doing so, it gained the respect and, therefore, the willing cooperation of the intelligence agencies.

But because the system rested on such stringent requirements, it was inherently fragile. This was demonstrated in the early 1990s when the process of effective oversight - particularly as far as the Senate was concerned - began to break down.

The first indications that the system was vulnerable occurred in 1991 when the nomination of Robert Gates as CIA director by the first Bush administration ran into heavy resistance. The White House reacted by informing Republican members of the SSCI that the president would "go to the mat" for Mr. Gates. This was a crucial moment in the history of intelligence oversight because the SSCI's Republican senators responded by rallying behind the nominee as a matter of political loyalty and obligation. Some of the Democrats on the committee responded in kind.

Unfortunately, the Gates hearing proved to be not an anomaly but a harbinger. When the SSCI chairmanship passed to Republican Sen. Richard Shelby of Alabama in January 1997, the committee's oversight capabilities were already crippled. Most of the professional staff of the 1980s had left, and they were replaced not by intelligence professionals but by political spear-carriers.

Equally important, a change of climate took hold. In place of an attitude that valued cooperation across political and institutional (legislative/executive) lines, a new "us vs. them" mindset became dominant. For the majority, intelligence came to be viewed as a club to wield against the other side of the aisle and against the administration.

The sad new world was put on public display during the 1997 confirmation hearings for Anthony Lake, nominated by President Bill Clinton to be CIA director. The hearings quickly degenerated into a political circus with vitriolic, partisan exchanges among committee members.

Since then, the deterioration in comity, professionalism and effectiveness has proceeded apace. An oversight system that was once a kind of national treasure is well and truly broken. The respect of the intelligence professionals has been largely forfeited.

Many of the hard questions senators will be asking about intelligence lapses are questions they should have been asking, and systematically examining, in months and years past. Unfortunately for us all, this breakdown comes at a time when a fully functioning intelligence system (including effective oversight) is not simply important - our collective lives may well depend on it.

Marvin C. Ott, a professor at the National War College, was a staff member of the Senate Select Committee on Intelligence. The views expressed are his own.