Coping with constitutional ambiguity. The US Constitution's division of foreign policy responsibility between president and Congress created a delicate balance that has been severely tested by the Iran-contra affair. Yet politicians and historians tend to agree that the system is wise and workable.

Tonight, on national television, President Reagan will make a speech and try to bury the Iran-contra hatchet with Congress. He could succeed. Yet the institutional tensions that helped create a crisis are almost certain to persist. On questions of foreign policy, Congress and the White House are often at odds, and successive presidents have struggled to assert their foreign-policy prerogatives against the right of Congress to say no.

But when Reagan administration officials endeavored secretly to aid the Nicaraguan contras, despite a congressional prohibition against US military support for the rebels, the struggle took a different turn. Laws apparently were violated, and a political scandal ensued that left the Reagan presidency seriously damaged.

``It was the horrible exception to the rule,'' observes Sen. Richard Lugar (R) of Indiana. It also rekindled an age-old debate over the proper balance between the executive and legislative branches in the making of foreign policy. The Constitution explicitly grants a president the right to receive ambassadors, negotiate treaties, and act as the commander in chief of the armed forces. But it gives Congress control over the federal purse strings, and the power to declare war.

The late historian Edward Corwin called this overlapping arrangement ``an invitation to struggle for the privilege of directing American foreign policy.''

Norman Ornstein, a congressional scholar at the American Enterprise Institute, calls it ``a deliberate construction by the Founding Fathers.''

In the 200-year history of the Republic, Congresses and Presidents have alternately been accused of wielding too much power at the other's expense. From the Roosevelt through the Johnson eras, Presidents dominated foreign policy making.

Since the withdrawal of the United States from the Vietnam war, however, Congress has been blamed for acquiring too much influence in foreign affairs. Frustrated by its inability to force an end to US involvement in an undeclared war, Congress took action to see that such an event would not happen again. Over President Nixon's veto, it passed the War Powers Act, a law that required a president to notify Congress whenever US forces were sent into an area where hostilities seemed imminent. After 90 days, the law said, the troops had to come home - unless Congress voted otherwise.

Next, stung by the ``dirty tricks'' of the Watergate scandal and revelations of ``rogue elephant'' operations by the Central Intelligence Agency, Congress stepped up its oversight over US intelligence operations, both foreign and domestic. It applied the brakes on covert operations abroad - passing, for example, the Clark amendment in 1975, which blocked US aide to rebels fighting Angola's Marxist regime.

The big question is whether such active congressional involvement in the nation's foreign policy has furthered or hindered US interests abroad. Critics lament what they see as Congress's overinvolvement in foreign affairs. The result, they believe, is vacillating policies crafted mostly to appeal to the broadest segment of the public, rather than to serve the national interest.

Congress's indecisiveness can be maddening. Consider the saga of contra aid. On different occasions, lawmakers voted yes, no, and maybe to President Reagan's contra-aid proposals. The resulting frustration, in part, led to the actions behind the Iran-contra affair.

After Congress acted to deny military aid to the contras, two White House aides, Lt. Col Oliver North and Rear Adm. John Poindexter, went underground to look for sources of support that would tide the contras over until, presumably, Congress changed its mind again and voted for a fresh infusion of aid. Later that year (1986), it did.

Diplomatic observers say skirmishes between lawmakers and presidents can undermine foreign confidence in an administration's ability to keep its word. To many of these observers, the Iran-contra hearings presented the picture of a government at war with itself. In their view, the hearings served as an impressive testament to US openness, but cast doubt on US trustworthiness.

Yet Congress's supposed obstinance is all too easily overrated. President Reagan has run into stiff congressional resistance on many of his foreign policy initiatives. Thus, his administration's 1982 proposal to sell advanced AWACS radar planes to Saudi Arabia ran into furious opposition in the Senate. Yet Reagan ultimately ruled the day, using his personal popularity and political clout to wrest further victories from Congress.

Reagan has exercised one of the most muscular foreign policies of the last decade and a half - sending US military advisers to El Salvador and US marines to Lebanon, launching an invasion of Grenada and an air strike on Libya, and asserting the US presence in the Persian Gulf war zone. Nevertheless, Congress has never forced Reagan, or his predecessors, to invoke the War Powers Act.

Congressional opposition to the contras led to the 1985 passage of the Boland amendment - the Reagan era's equivalent of the Clark amendment - which prohibited the expenditure of any appropriated federal funds to help aid the rebels.

The apparent violation of the Boland amendment by former White House officials, who planned to sell arms to the Iranians and divert profits from those sales to the contras, lay at the heart of the Iran-contra affair.

But a year after Congress passed the Boland amendment it granted the Reagan administration's request for $100 million in military and nonmilitary aid to the contras. Earlier in Reagan's presidency, Congress also repealed the Clark amendment, thus permitting US aid to flow to the Angolan rebels.

``The fact is, this President has gotten just about everything he wanted,'' says Sen. John Kerry (D) of Massachusetts, a member of the Senate Foreign Relations Committee.

On a number of important occasions, Congress and Reagan have worked with, rather than against, each other in the foreign policy arena. Last week, the President and House Speaker Jim Wright (D) of Texas agreed on a major peace proposal for Central America.

At other times, Congress has brought the President around to its point of view. Congress is generally credited with motivating Reagan to hasten the fall of former Philippine President Ferdinand Marcos. At the same time, lawmakers forced the administration to accept stiffer sanctions against South Africa than it wanted.

John Norton Moore, a constitutional law professor at the University of Virginia, contends that the two branches ought to avoid squabbling over constitutional prerogatives and seek instead to find more cooperative arrangements.

All of which may be easier said than done.