In thinking about the U.S. intelligence-gathering system and its 11 components, the congressman said, the difficult part is pinpointing "the moment when the process of understanding a situation slides into the process of changing it."

Like many members of Congress concerned about the uses and control of U.S. intelligence, this Democratic politician sees a great deal at stake in decisions involving the House and Senate intelligence committees that await the convening of the 99th Congress in early January.

Proposed and pending changes in committee membership and structure could alter drastically congressional oversight of the U.S. intelligence-gathering system.

The outcome could have manifold implications for U.S. policy in Central America, the debate on how to combat international terrorism and the nature of the U.S. presence abroad.

If current rules hold, the 15-member Senate Select Committee on Intelligence will lose nine members -- including Chairman Barry Goldwater (R-Ariz.) and Vice Chairman Daniel Patrick Moynihan (D-N.Y.) -- who have served on the committee for at least eight years.

The House Permanent Select Committee on Intelligence, with 14 members, is expected to lose eight members who have served at least six years, including Chairman Edward P. Boland (D-Mass.).

If these shifts occur, they would bring in Sen. David F. Durenberger (R-Minn.) as chairman and Sen. Patrick J. Leahy (D-Vt.) as vice chairman on the Senate side, and Rep. Lee Hamilton (D-Ind.), an articulate and widely respected moderate, as chairman on the House side.

All are expected to be much more critical of Reagan administration policies than their predecessors, and all favor stressing intelligence-gathering and analysis over covert action -- such as using paramilitary forces or trying to undermine foreign governments.

Durenberger has served notice that he will try to block the administration's program of aid to the "contra" rebels fighting the leftist government of Nicaragua. Hamilton has been a leader of opposition to the aid in the House.

But more than half the committees' members would be new at understanding the complex intelligence secrets of the nation, at a time of increasing world violence and pressure on U.S. policy abroad.

They also would be unaccustomed to deciphering the elliptical style of agents who divulge those secrets, even to members of Congress, with great reluctance.

Expressing those concerns, a special Senate committee on reorganization, headed by Sen. Dan Quayle (R-Ind.), recommended that the House and Senate committees be merged into a joint committee on intelligence along the lines of the old Joint Committee on Atomic Energy.

It also has called for a two-year extension before enforcing the rule that limits senators to eight-year terms on the intelligence committee, thus all current members would be allowed to remain on the panel at least two more years.

Quayle argued that a joint committee would curb leaks of intelligence information (although none has been blamed on the committees), streamline decision making and eliminate repetitive testimony. The Central Intelligence Agency long has backed this approach.

But Goldwater has opposed it, noting that it was members of the Joint Committee on Atomic Energy who improperly revealed on television in 1949 that there had been progress toward creation of a hydrogen bomb.

Sen. Robert J. Dole (R-Kan.), the incoming Senate majority leader, will have power to influence the outcome, but has not expressed his views on the issues.

The two intelligence committees were made permanent in 1975 under Senate Resolution 400, which set two goals: to make the intelligence agencies accountable to Congress and to write charters that would bring each intelligence agency under a legal code.

The charters have not been written. Hearings in 1977 and in 1979 broke down over what kinds of limits Congress ought to set on the units, which include the CIA, the Federal Bureau of Investigation, the National Security Agency, the Defense Intelligence Agency and the security arms of the Departments of Energy, the Treasury, State and the four branches of the armed services.

House proposals this year for new curbs foundered after hearings at which witnesses generally agreed that Congress' only real stick is the same as its carrot: the budget process.

The agencies, therefore, still operate on general guidelines given in directives from each president. The current one, executive directive 12333, is about 25 pages long and was issued Dec. 8, 1981, by President Reagan.

There is much doubt that the agencies are as accountable to Congress as a 1980 law requires. Goldwater and Moynihan complained that they were not informed that the CIA had mined the harbors of Nicaragua last year. After 261 servicemen died in bombing attacks in Lebanon, intelligence committee members said they felt misled over repeated agency assurances that security was adequate at U.S. facilities abroad.

Reagan's directive bans political assassinations, but the CIA prepared a guerrilla manual in 1983 recommending that U.S.-backed rebels in Nicaragua use violence to "neutralize" Nicaraguan officials.

"A lot of us worked in a bipartisan way to beef up the intelligence agencies over the past seven or eight years, and we did it on the assumption there are checks and balances so they can't go off and do whatever they want. This manual operation raises very fundamental questions about whether those checks and balances are there," Leahy said in a recent interview.

"It also puts into question other operations around the world . . . . There should be a clear-cut chain of command that responds to clear-cut legal mandates," he said.

Only four members of the Senate intelligence committee have said farewell. Goldwater will depart to assume the chairmanship of the Senate Armed Services Committee; Sen. Walter D. Huddleston (D-Ky.) lost a reelection bid, and Moynihan and Sen. Daniel K. Inouye (D-Hawaii) have said they will abide by the 1976 rule limiting service on that committee to eight years.

Five others are scheduled to depart under that rule: Sens. John H. Chafee (R-R.I.), Jake Garn (R-Utah), Richard G. Lugar (R-Ind.), Malcolm Wallop (R-Wyo.) and Joseph R. Biden Jr. (D-Del.). But all except Biden have asked that the eight-year rule be waived or modified.

If it is, Chafee will succeed Goldwater as chairman. Understandably, Durenberger, who otherwise will be chairman, is opposed to a waiver.

Sources close to both senators said either would hold more open hearings than Goldwater did. Durenberger opposed CIA efforts to exempt itself totally from disclosures under the Freedom of Information Act and otherwise has complained of agency evasiveness; Chafee wrote the Intelligence Identities Protection Act that makes it illegal to reveal the names of agents.

The eight-year rule, designed to curb chumminess between the regulators and the regulated, repeatedly has been honored in the breach. Several senators assigned staggered departure times beginning in 1977 have remained, in part because no one objected.

But now there are objections.

Sen. William S. Cohen (R-Maine) proposed a modified rule that would allow four members, two from each party, to remain as committee members -- but not leaders -- for an additional two years in order to ease the transition. But this, Cohen said, would have to be approved by the Senate.

"There have been times when no one wanted to get on the committee," said an aide to one Republican senator. "That's one reason the eight-year rule was waived. I don't think that's going to be the case this time.