As the shelling of United States marines in Lebanon continues, Congress faces a major test of whether it has the power, or the will, to exert authority over sending troops abroad.
The lawmakers head into the second week of their new session with little prospect for quick action. The House of Representatives had scheduled a vote on Lebanon for tomorrow, but the leadership cancelled it because Congress and the White House are still deadlocked over the issue.
For the most part, debate over Lebanon is going on behind closed doors, as members take care not to appear to be pulling the rug from under the 1,400 marines in the multinational peacekeeping force in Beirut and their 2,000 reinforcements waiting offshore.
But it is becoming increasingly obvious that a constitutional struggle over the powers of the executive and legislature is going on. For the first time since the end of the Vietnam War, a serious question has been raised over how much war power Congress has.
''How, when, why, and where we go to war, and who decides it, goes to the very foundation of the nation,'' said Sen. Thomas F. Eagleton (D) of Missouri, a leading advocate for more congressional say over troops in Lebanon. Senator Eagleton said in an interview that talks have hit an impasse over use of the 1973 War Powers Resolution, an act of Congress that prevents a president from ordering a massive troop buildup as in Vietnam.
The White House is ''still adamant'' against using a key section of that resolution, said the Missouri Democrat, describing a meeting Friday with with White House chief of staff James A. Baker III, Senate majority leader Howard H. Baker Jr. (R) of Tennessee, and Senate minority leader Robert C. Byrd (D) of West Virginia.
The contested section, known as ''4-1-A,'' of the 1973 War Powers Resolution calls for a president to notify Congress that he is sending troops into a hostile area. By law, Congress must then authorize the deployment, or else the president must bring the soldiers home after 60 to 90 days.
Senate Democrats insist the section must be invoked. ''If this isn't hostilities, what is?'' Eagleton said of Lebanon, where four US marines have been killed in recent weeks.
At issue is not so much whether to support the troops in Lebanon. Senator Byrd told White House officials at the meeting Friday that ''a majority of Democrats thought the marines should remain in Lebanon for a reasonable time,'' Eagleton recalled.
Democratic senators, who are leading the effort to regain the initiative in Lebanon, are trying to attach strings and a time limit to US military involvement there. But Eagleton concedes there isn't enough support in Congress to force President Reagan to invoke the time limit of the War Powers Resolution.
Trying to break the stalemate, Senator Baker has said he will introduce a compromise this week to give Congress some authority in US policy in Lebanon. Such a plan might mention the War Powers Resolution, but probably wouldn't go as far as Senate Democrats want.
Meanwhile, House Democrats are trying to play down the issue. ''The point is not to get into a confrontation,'' says a Foreign Affairs Committee staff member. ''To have a big shoot-out here would be jerking the rug out from under the fellows who're being shot at.''
The staffer also said that House Democrats would be willing to go along with Mr. Reagan for ''at least for some limited period,'' and that the question is ''how to do it without losing'' the congressional role as set forth in the War Powers Resolution.
It is still unclear what that role is. Lawyers and some members of Congress still debate whether the act is constitutional, and the courts have yet to rule on it. While presidents since Gerald Ford have filed ''war powers reports'' on occasion, they have also found ways to work around the act. President Carter ordered troop movement in South Korea in 1976 and aided in the transport of European troops to Zaire in 1978 - relatively minor incidents - without issuing war powers reports.
''This is it's (the resolution's) first real test,'' says the House staff aide of the Lebanon question. Reagan has filed three reports on sending troops to Lebanon, but he has not reported that they would be in hostilities or imminent hostilities, thus avoiding the 60- to 90-day limit. Now hostilities are apparent.
''If the War Powers act isn't triggered in this instance,'' said Eagleton, ''then the War Powers act is pretty close to a dead letter.''