Iran-contra affair prompts 200-year-old question: Who's in charge of foreign policy? The Constitution sets up some guidelines. But our dealings with other countries are increasingly complex. As a result, the lines that separate Congress's role from that of the president have blurred.

THE Iran-contra affair has put a new focus on the historic question of the balance of power within the United States constitutional system. From the start, Congress has been a reluctant partner in the Reagan administration's efforts to fund the contra rebels, who are fighting to overthrow the Nicaraguan government. But that opposition has grown since the White House disclosed last November that the administration had sold arms to Iran covertly, and that certain officials later attempted to divert profits from the arms sales to the contras without going through Congress.

Congressional committees are investigating whether any of the administration's secret activities were illegal or a violation of constitutional checks on executive authority.

``We need separation of powers,'' stresses Alfred Rubin of the Fletcher School of Law and Diplomacy at Tufts University. ``We don't have a parliamentary system. And we don't have a king. We restrict what the President can do as a party leader. And we don't want [him] to co-opt Congress.''

Interviews with constitutional lawyers, historians, political scientists, and foreign policy specialists show that:

The president - as provided by the Constitution - continues to be the central figure in the design and practice of foreign policy. But the Iran controversy has heightened debate over how best to check presidential power and provide accountability for Oval Office action.

Although Congress wants greater oversight of the US's international actions, it has exhibited little willingness to show a strong hand in crises. Critics say this is because legislators worry that they might not be reelected if a congressional policy proved unpopular.

The Supreme Court, and lower courts, shy away from reviewing most disputes between a president and Congress over foreign affairs. When they do accept jurisdiction, they lean heavily toward presidential authority. The high court, one expert points out, is presidentially appointed, and its members often come from executive backgrounds. Chief Justice William Rehnquist, for example, served in the Nixon administration's Justice Department.

Government spokesmen say they heed the principles of international law. But the Reagan administration has often backed away from allowing international bodies such as the World Court and the UN to test the legality of its policies, particularly if it appeared that those judgments would conflict with what the administration regards as national priorities.

Studies abound that recommend ways to alter constitutional structures and procedures for carrying out foreign policy. But actual reform, through constitutional amendment or even changes in practice, is a slow process.

The tug of war between the President and Congress over authority in international affairs dates back to George Washington. But the intricacies of modern foreign policy initiatives, particularly those involving less than all-out war and responses to terrorism, have greatly blurred lines of jurisdiction between the executive and legislative branches.

Actually, constitutional analysts say, the lines were never clear. The Founding Fathers set some specific guidelines in the Constitution. They made the president commander in chief of the Army and Navy. The chief executive was also given the power to conclude treaties and appoint ambassadors, with the consent of the Senate. Congress, on the other hand, has the authority to declare war and holds the purse strings that govern the funding of US foreign military involvements.

But even these constitutional mandates have been altered by practice and have been interpreted differently by various presidents. Circumstances have determined how the nation views the use of war powers, says Michael Belnap, professor of history at the University of Georgia. Professor Belnap explains that President Roosevelt exerted extensive ``war powers'' up to, and during, World War II - ``reducing congressional declaration of war [against Japan] to little more than a formality.''

And President Truman, without consulting Congress, writes Belnap in a bicentennial chronicle of the American Political Science Association, simply ordered US troops into combat against North Korea after the latter's attack on South Korea. He later explained that ``he had to act as commander in chief.'' PRESIDENTS Johnson and Nixon were sharply criticized for sending military units to Vietnam without congressional authority. This action triggered the War Powers Resolution - passed by Congress over President Nixon's veto in 1973. The resolution requires the president in every possible instance to consult with Congress before introducing armed forces into hostilities.

Belnap holds that Mr. Johnson and Mr. Nixon, accused of abusing presidential war powers, were no more guilty of this charge than were Roosevelt and Truman. ``They behaved no more unconstitutionally than their predecessors. They were just less successful,'' he says.

Many foreign policy matters fall into what Louis Henkin of Columbia University Law School calls the ``twilight zone'' of jurisdiction, referring to foreign-policy situations where congressional and presidential authority overlap and where its distribution is unclear.

Professor Rubin says that a clear distinction must be made between the president's role as commander in chief and his warmaking power. Rubin explains that the Constitution gives Congress a clear check on the latter through the power of the purse.

He indicates that in the current Iran-contra crisis, if money was acquired without congressional authorization - with no accounting for receipts - a serious question must be raised over possible abuse of executive authority.

``The big problem in recent years is the exaggeration of presidential power,'' says Pat Holt, former chief of staff of the Senate Foreign Relations Committee. ``Congress has the wherewithal to use [power]. They like the prestige of being consulted. But they're usually willing for the buck to stop on the president's desk.''

``[Reagan] speaks for the nation,'' says Henkin. ``But it doesn't give him authority necessarily to either send troops or to spend money. To spend money he needs congressional authorization. He can't spend a penny which isn't authorized by Congress. That clearly [flies] in the face of the Constitution. And the same is true about sending troops.''

In recent years, Congress has occasionally flexed its muscles to curb executive action. For instance, it invoked the War Powers Resolution to compel President Reagan to withdraw US troops from Lebanon. The President, however, said he would have eventually done so anyway. And now federal lawmakers are conducting a full-blown investigation into the Iran-contra affair to determine, among other things, if there was presidential wrongdoing. HISTORICALLY, neither the US Supreme Court nor the World Court in The Hague has played a great role in determining the nation's foreign policy. ``The [US] courts duck political questions,'' says political scholar Thomas Cronin at Colorado College. And, he explains, the Supreme Court has tended to regard disputes between a president and Congress over foreign policy as a political issue. For example, when 29 congressmen brought suit against President Reagan in 1982 over Central America policy, the high court determined that Congress had the resources to investigate the matter itself.

The US snubbed the World Court when it refused to recognize the latter's jurisdiction in 1984 in cases involving Central America.

``Basically, it [the World Court] is an irrelevance,'' says legal expert Bruce Fein of the conservative Heritage Foundation in Washington, D.C. ``We pay attention when we want to.''

Professor Rubin stresses, however, that the US has a ``great stake'' in the maintenance of international law. ``We can't isolate ourselves from the world. We can't go it alone,'' he adds. ``International law is not only a question of rights and duties but a question of distribution of authority.''

Through the years, attempts to change the US Constitution have included recommendations that would affect foreign policy. For example, the Committee on the Constitutional System, chaired by former White House counsel Lloyd Cutler, recommended last month a relaxation in the requirement that treaties be approved by a two-thirds vote of the Senate - either by a constitutional amendment permitting ratification by a majority of each chamber or reducing the required Senate vote to 60 percent.

Presidential scholar James MacGregor Burns at Williams College would require a president to choose members of Congress for his Cabinet. Such appointees could not be dismissed, or ignored, because they would have been elected by the public. ``We need to establish institutionally close to the president people who will say `no' with authority,'' Professor Burns says.

Thomas Cronin stresses the need for greater presidential, congressional, and public responsibility. He scores recent chief executives (among them Johnson, Nixon, and now Reagan) for failing ``to set a tone for the foreign policy of the nation'' rather than pursuing their own political agendas.