President Reagan now faces in Lebanon the greatest crisis of his presidency. In the short run, he appears to lean toward reinforcing the American commitment in Lebanon following the Beirut bombings that took the lives of at least 135 United States marines and 27 French soldiers.
But most analysts predict that in the long run, strong pressure from the public and Congress will force the President to focus more intensively than ever before on a negotiated way out of the Lebanon crisis. This will require a skillful use of both force and diplomacy.
Reagan's response to the crisis could determine whether he is reelected or not, if he decides to run for a second term.
A recent poll showed that half of all Americans oppose the involvement of US marines in Lebanon and that even more Americans fear that the nation will be drawn into a major war there. Earlier polls have indicated that Mr. Reagan was not making a great enough effort to reach arms control agreements with the Soviet Union. A peaceful outcome in Lebanon could do much to overcome public fears about the President's approach to the issue of war and peace.
In his initial public reaction Sunday to the attack on the marines, Reagan denounced the ''bestial nature'' of the bombings and declared that they cannot drive the Americans out of Lebanon. He reasserted that Lebanon was a nation of ''vital and strategic'' importance to the US. Defense Secretary Caspar W. Weinberger said the US was looking for ways to reduce the marines' vulnerability , including a possible new location for them.
From Congress, there was little sentiment in favor of an immediate pullout of the marines. Indeed, some members favored a short-term reinforcement of the US contingent in order to protect the marines who are still there. But Sen. Sam Nunn (D) of Georgia, an influential member of the Senate Armed Services Committee, seemed to express widespread sentiment when he said the marines' mission needed to be more precisely defined.
Appearing Sunday on the NBC television program ''Meet the Press,'' Senator Nunn proposed that the marines be reinforced in the short run to protect the remaining force. But in the long run, he said, United Nations peacekeeping forces ought to replace the marines. He asserted that the current American policy of trying to clear all foreign forces out of Lebanon was ''mission impossible.''
''We are no longer a deterrent force in Lebanon,'' Nunn said. ''Our marines are hostages.''
Whatever Reagan decides to do, he will have to consult more closely with Congress than in the past. Sen. Charles McC. Mathias Jr. (R) of Maryland, a member of the Senate Foreign Relations Committee, said Sunday that the President could not send large numbers of reinforcements to Lebanon without the concurrence of the Congress.
Senator Mathias and others suggest that Reagan must now broaden his efforts in Lebanon to include a better-defined, long-range peace plan for that nation and for the region as a whole. This would have to include a renewed effort to deal with the Palestinian problem, he said.
The former US ambassador to Egypt, Hermann Eilts, has just returned from a two-week trip to the Gulf, where, he said, anti-American sentiment is now strong. Mr. Eilts said that the Lebanon ''sideshow'' has absorbed US attention to the point where other aspects of Middle East policy have been neglected.
''There are many cross-pressures on the President,'' says Stephen Wayne, a political scientist specializing in the presidency at George Washington University. ''In the short run, the President must do something tough and demonstrative which will have the country's support. But he cannot burn his bridges and must move toward negotiation.''
''The same thing is true in Central America,'' Mr. Wayne said. ''Nicaragua has proposed terms. So my guess is that the President could be forced into a posture of negotiation in both areas. Reagan needs to be reelected as a peace President.''
White House spokesman Larry Speakes said, meanwhile, that intelligence reports pointed to an Iranian-supported group in Lebanon as the possible perpetrator of Sunday's bombings. But he said that there was no hard physical evidence from the scene of the bombing at the Marine headquarters in Lebanon to prove that the Iranian-backed Shiite Muslim group was involved. This same group is suspected of bombing the US embassy in Beirut last April.
Some experts have suggested that retaliation against Iranian troops present in Lebanon might be called for. But the number of Iranians there is relatively limited. Such retaliation might call for more pinpoint precision than is possible in the midst of Lebanon's crazy-quilt factions.
If Reagan decides that a stronger negotiating push is called for, he now has a national security team with considerable experience in dealing with the Lebanese and Syrians. Robert C. McFarlane, his new national security adviser, recently spent three months as special envoy in the region, negotiating with all the parties to the Lebanon conflict. Secretary of State George P. Shultz presided over talks between the Israelis and Lebanese that led to an Israel-Lebanon agreement last May.
''A positive aspect to all this is that the US decisionmaking apparatus is in a position where policies can be made and coordinated,'' says Charles F. Doran, a political scientist at the Johns Hopkins School of Advanced International Studies. ''The difficulty is where to go. Reagan must react with firmness and bold action. But he cannot get trapped into a deeper guerrilla involvement. That is the dilemma.''
Administration officials are acutely aware of the political fallout that can occur if a foreign policy goes sour: President Carter was defeated in 1980 not only because of the poor state of the economy but also because of the public perception that he did not know how to cope with the Iran hostage crisis. But in foreign policy terms, the Iran hostage crisis was a relatively limited incident, involving a limited number of Americans in a fixed location. In Lebanon, experts say that American credibility is at stake throughout the Middle East.
Democratic presidential candidates Alan Cranston of California, Gary Hart of Colorado, and Ernest Hollings of South Carolina said Sunday that Reagan is violating the War Powers Act by keeping the marines in Lebanon.
''President Reagan should report to Congress under the War Powers Act, as he should have done in the first place,'' Senator Cranston said.
''He should either come to the Congress or get out,'' said Senator Hollings.