The Reagan administration has reinforced its commitment to Lebanon - rather than pulling back, as advised by some congressmen. On Tuesday, one day after two American marines were killed in an upsurge of fighting in the Lebanese capital, the administration ordered the United States aircraft carrier Eisenhower to waters off Beirut as a show of support.
At the same time, the administration refused to withdraw the marines from Lebanon, as some US senators and representatives have proposed, despite the worsening security situation there. Administration officials argued that a withdrawal would mean a collapse of United States credibility in the region.
Having made a commitment to deploy the marines as part of a multinational peacekeeping force a year ago, US officials say, the US cannot pull out at this critical juncture. Such a move, they say, would shatter the confidence of the government of Lebanese President Amin Gemayel.
''A pullout of the marines would remove the US from the Middle East,'' says one administration official. ''It would be an abdication of responsibility. It would mean the end of the Gemayel government. . . .''
On the ground in Beirut, the marines and the other members of the international peace force came under attack again Tuesday. At time of writing, five French soldiers had been killed and all the marine positions in the southern sector of the Lebanese capital were reported to be under fire and responding with howitzers and Cobra helicopters. The fighting was said to be some of the worst since the 1975-76 civil war.
Meanwhile, the administration sees the resignation of Israeli Prime Minister Menachem Begin as making little difference in the Lebanon situation. Mr. Begin's successor, whoever he may be, will probably continue Begin's policies, officials here say. And even if the opposition Labor Party eventually takes power in Israel, it would move more cautiously before making changes in Lebanon than its current rhetoric suggests, officials say.
Despite the latest turmoil and fighting in Beirut, administration officials say that President Reagan's special envoy, Robert C. McFarlane, has made progress over the past few days in bringing about a reconciliation in Lebanon between President Gemayel and the Druze forces headed by Walid Jumblatt. In the administration's view, such a reconciliation would require a commitment from Gemayel, assuring the safety of the other Lebanese factions, including the Druzes, and an offer to those factions of a greater share in government responsibilities.
Israel has agreed to a request from President Reagan to delay its planned withdrawal of troops from the Shouf mountain region southeast of Beirut, giving McFarlane more time to arrange a political deal between Gemayel and the Druzes. US officials hope this will help defuse tensions in the Shouf between Maronite and Druze militias, and possibly avert an outbreak of full-scale civil war.
Some administration officials charge that Syria is playing an obstructionist role in the Lebanon situation by encouraging groups that oppose moves by the Lebanese Army into the Shouf region.
The fear of some US senators and congressmen is that the Lebanese government is incapable of controlling the various factions. As they see it, the situation is growing more unstable. Increasing instability means greater danger to the marines.
Several senators and congressmen now propose that President Reagan leave it to Congress, under the War Powers Act, to decide whether the marines should stay or go. But Reagan has decided merely to report to the Congress on the marine casualties that were incurred Monday, without invoking the provision in the act that would call for a congressional move. That provision requires that American forces be withdrawn within 60 days unless the Congress enacts special legislation authorizing them to stay.
Not all of those senators and congressmen who propose congressional action are advocating that the marines be withdrawn. Sen. Charles McC. Mathias Jr., for example, a Maryland Republican who serves on the Senate Foreign Relations Committee, says that he would not call on the President to pull the marines out at this point. But Mathias argues that the loss of two marines on Monday brings the question of the marines' presence in Lebanon within the responsibility of the legislature.
Sen. John Glenn of Ohio, a contender for the Democratic presidential nomination, issued a statement saying he favored keeping the marines in Lebanon. But Glenn argued that the situation now requires that Congress vote on the issue.