IN THE AFTERMATH OF the presidential excesses that were blamed for our Indochina debacle and Watergate agonies, Congress reclaimed for itself a substantially greater role in determining what constituted the national security of the United States. No longer would our legislators, as they generally had during the cold war years, permit the executive to determine, virtually on its own, when and how this nation would exercise its power.

Naturally, the main question which Foreign Policy by Congress seeks to explore is whether the system is working better with the new balance of power than with the old. This important book's answer is that congressional foreign policy reflects the virtues and flaws of democracy in full expression. The 535 members of Congress, acting out of vastly different experiences, interests and influences, assure that anything we do will be openly and extensively considered before measures are taken. No longer possible are the secret destabilizations of governments, like that of Salvador Allende's Chile, or the secret bombings, like that of Cambodia in 1968.

American foreign policy, as it heads into the 1980s, is certainly more responsive to national sentiments than it was in the heyday of White House dominance -- and that is an unquestionable plus.

The problem, however, is that deciding what is best for the country isn't always the same as choosing what is most expedient or popular at any particular moment. The restraints placed on the president since the onset of congressional policy-making in 1975 have significantly reduced U.S. flexibility in coping with a world changing so fast that yesterday's adversary might well be today's potential friend. As authors Thomas M. Franck, professor at New York University School of Law, and Edward Weisband, professor of political science at the State University of New York at Binghamton, observe: "Since Congress is primarily a legislative body, foreign policy-making by Congress usually means the making of policy by laws. Characteristic of laws is predictability, a removal of uncertainty.Yet in foregin relations, a degree of flexibility may be a valuable asset."

For example, because of past wrongdoing by their leaders, the United States is prohibited from aiding Uganda and Somalia although both countries certainly qualify for assistance now. Mozambique is a Marxist, pro-Soviet nation, but does a thriving subterranean trade with South Africa and might well be weaned away from Moscow -- if we were in a position to try. But because of congressional restrictions, administration options are sharply reduced.

Another difficulty is that Congress can sometimes display the worst of American jingoism in it deliberations. Even with the expanded staffs of the past decade, the apparent ignorance of some members on certain foreign policy issues is astounding. Take the Panama Canal Treaties. Franck and Weisband provide a vivid case history of how the controversial treaties wended their way through the Senate. They explain the fine arts of bargaining, beseeching and blandishment required to win over th necessary majority.

I would have added that the terms of the floor debate, in which Panamanian leader Omar Torrijos was roundly vilified, were neither illuminating nor beneficial to American interests. To hear Torrijos denounced as a tinpot dictator and Castroite, one would have thought Panama was a veritable Cuba. In fact, it has one of the most freewheeling economies in Latin America, is a favorite of big banks and is so closely aligned to the United States that the Panamanian currency is dollar bills.

How ironic, after all the abuse directed at him, that when the U.S. was casting about for someone to take the deposed shah of Iran, it was Torrijos who came to the rescue.

The Senate, of course, advises and consents on treaties even in periods when its foregin policy role is far less important than currently. But as Franck and Weisband make clear, the assumption of major responsibilities in directing national security imposes a demand for expertise in Congress not yet always fully met.

A further danger to the enhanced congressional role is the place of politics -- trade offs, compromise and promises made for the essential purpose of breaking stalemates. In one of its initial, bruising battles with the White House, Congress finally reached a politically efficient compromise with the executive when it ordered that the bombing of Cambodia end on a statutory date 45 days hence. The deal was senseless because it neither spared the Cambodians additional suffering nor persuaded the communists to begin negotiating, since they knew exactly when the bombing would cause. As Franck and Weisband remark: "The 'let's split the difference down the middle' type of legislative compromise, so much a part of the way Congress and the President traditionally interact on domestic issues, can produce the worst of all possible worlds when applied to foreign policy-making."

There is also the influence of lobbyists. Congress is, by and large, more susceptible to pressure from outside interests than the more remote executive. Franck and Weisband point out that more than 500 corporations operate Washington Lobbies, a five-fold increase in the past decade. In addition, there are more than 1500 trade and professional associations that maintain headquarters here, at least partially because of Congress.

If Congress were capable of collective action beyond the reach of special interests, it is conceivable that the policies and principles pursued would be more universally irreproachable than they are now. But ours is a system in which people have to get re-elected. Individual interests, therefore, can have considerable and profound effect on foreign policy. The effectiveness of the Jewish and Greek lobbies, to take two examples cited by Franck and Weisband, has at times surpassed their salutary influence on U.S. actions.

On the whole, though, the authors feel that Congress is doing progressively better in carrying out its new duties. "Defying its legions of detractors," they write, "and despite a few wild swings, Congress has not struck out. This revolution began . . . as have all others: with a series of sharp destructive attacks on the old order. But then Congress turned to reconstruction, devising an elaborate new system of rules of designing policy by codetermination, rather than by dominion of one branch ovr the other."

Pendulums also tend to swing, however, and it is possible that we are on the verge of another movement. The national mood seems to be changing from one of reserve and something approaching neo-isolationism to a sense that we must be more assertive again in protecting our security. In the past such periods have usually meant greater authority in the executive.

The burden of argument in Foreign Policy by Congress is that the legislature has established itself firmly and is better equipped to maintain its place in decision-making. Both scholarly and up-to-date, lucid yet densely packed with facts and analysis, it is a book well worthwhile for better understanding a crucial factor in our national politics.