Washington — With American credibility throughout the Middle East at stake, President Reagan has decided to make a new plunge into Lebanon peacekeeping.
Officials say the President still pins a great deal of hope on the Lebanese themselves, and their well-known resilience. But in the wake of the assassination of President-elect Bashir Gemayel, the Israeli drive into west Beirut, and now the massacre of unarmed Palestinian refugees, the Lebanese government issued a new call for limited, temporary international help. At press time, it appeared that President Reagan decided to respond by sending a contingent of US marines back into the beleaguered city.
Despite the obvious dangers involved, President Reagan decided to throw his support behind proposals for a new international peacekeeping role in Lebanon apparently because of the equally great dangers of US inaction. Not the least of those dangers is a complete loss of American credibility among the Arab nations should the administration fail to act. The Reagan administration had given its guarantee that innocent noncombatants would be protected once the forces of the Palestine Liberation Organization were withdrawn from Beirut, but such protection was lacking.
No one denies that US involvement in a new Beirut peacekeeping effort would entail the risk that US soldiers might be endangered. The US Defense Department has been most reluctant to commit itself to playing any role in what one Pentagon official called the ''Lebanon morass.'' But the risks were seen here as considerably reduced by the fact that the Lebanese themselves, despite multiple crises, seemed to be pulling together in an orderly fashion:
* The Lebanese have called for an election on Sept. 21, and most observers were predicting that Amin Gemayel, brother of the slain President-elect, will easily win the vote, thanks in part to heavy support from the Muslim community.
* Units of the Lebanese government army were reported to be taking over some positions in west Beirut from Israeli units, a sign that the Israelis may be willing to make gestures in order to redeem their tarnished image.
''The Israelis have been turning various positions over to the Lebanese army . . . ,'' said Ghasan Tueni, Israel's ambassador to the United Nations. ''Twenty-five UN observers have already gone to Beirut. So there is movement.''
''We are now hoping that increased US involvement will provide the credibility that is needed,'' the Lebanese ambassador told the Monitor. ''A US commitment is essential . . . .''
Lebanese officials said that the US role was required in part because of heightened tensions and in part because some Lebanese factions, most notably Muslim factions, do not trust either the Israelis or the Lebanese army.
Robert Basil, chairman of the policy committee of the American-Lebanese League here, spoke on the morning of Sept. 20 by telephone with Amin Gemayel.
''I think Amin would support that,'' said Mr. Basil when told that the US intended to participate in a new multinational force alongside France and Italy. It was US Marines and French and Italian soldiers who oversaw the withdrawal of Syrian and PLO units from Beirut earlier this month.
Basil said that Amin Gemayel strongly denied that any forces associated with his brother's Christian militias had engaged in the massacre of Palestinian refugees which occurred last week. Most reports received here do not point to those militias as the perpetrators of the massacre but instead to the forces of Maj. Saad Haddad, an Israeli-backed Lebanese army renegade who directs a mixed force of Christians and Shia Muslims.
A new peacekeeping force was needed, Basil said, ''because nobody can be sure in this volatile situation that the Lebanese army can handle the whole situation.''
According to administration officials, the turning point for President Reagan in his decision to send the marines back to Lebanon was the refugee camp massacre. On the night of Sept. 19, Secretary of State George P. Schultz informed Moshe Arens, Israel's ambassador here, that the US was considering the possiblity, among other options, of returning the marines as part of a multinational peacekeeping force. Officials said Mr. Schultz also told Mr. Arens that President Reagan was continuing to insist that the Israelis withdraw their forces from west Beirut. On Sept. 20, Reagan met for about an hour with Schultz, Defense Secretary Caspar Weinberger, Vice-President George Bush, national security adviser William P. Clark, and the President's special Middle East envoy , Philip Habib. It was later announced that Mr. Habib would be returning to the Middle East shortly.