Reagan administration officials admit that sending US marines to Beirut would involve risks, but argue that it could prove a key to success in the difficult Lebanon negotiations.
The Palestine Liberation Organization's initial rejection of the use of US troops in an evacuation from Beirut is not taken by officials here as the final word from the PLO. But these officials acknowledge that much more than this PLO rejection must be overcome before a Lebanon agreement is reached in the face of what appears to be an Israeli-imposed deadline.
In a breakfast meeting with reporters July 7, the US defense secretary, Caspar W. Weinberger, said that Philip C. Habib, the special American envoy in Lebanon, had only until July 11 to conclude ''hideously difficult'' negotiations. Mr. Weinberger asserted that such a deadline had been set last week by the Israeli Cabinet. There was no immediate confirmation of this from Israel.
Weinberger said the proposal to use American troops to help evacuate PLO guerrillas from Beirut - a proposal to which the United States has agreed in principle - was only one of about a dozen pieces which had to be fitted together into a delicate, and as yet incomplete, agreement. He hinted that one thing that would be part of the final outcome for Lebanon would be the prospect of increased American economic aid to that nation.
''If all foreign forces are removed, we would perhaps want to help restrengthen their economy,'' said Weinberger.
One key question remaining is how far the administration will go in committing itself to helping find a political solution to the Palestinian problem. Saudi Arabia, which has been playing a leading role in helping persuade the PLO to remove its forces from Beirut, has been pressing, along with France and Egypt, for American help in finding a homeland for the Palestinians.
Saudi Arabia's foreign minister, Prince Saud al-Faisal, is expected to arrive in Washington shortly to explain the Arab position on Lebanon. In the words of one source close to the Saudis, the prince will ''try to link a solution in Lebanon with progress on the Palestinian problem.''
The Reagan administration is committed to pushing for an Egyptian-Israeli agreement on ''autonomy'' for the more than 1 million Palestinians living on the West Bank of the Jordan and in the Gaza district. But it has never gone as far as President Carter did in advocating a Palestinian homeland. The concept of a homeland has been tantamount in Israeli eyes to a Palestinian state, an idea which the Israelis have repeatedly rejected.
Defense Secretary Weinberger would say only that ''ultimately'' President Reagan wants a solution to the Palestinian problem, thus indicating that the administration would not consider the departure of the PLO leaders and fighters from Lebanon to be the final answer to the problem. Weinberger also stressed, more than once, that it was in the interests of the United States to have many friends in the Middle East. By this he apparently meant moderate Arab regimes in addition to Israel. He said that both Saudi Arabia and Jordan had strongly supported Philip Habib's negotiating efforts in Lebanon.
The Saudis have been arguing that the PLO must be preserved as a political entity. The destruction of the current PLO leadership by Israel through an attack on West Beirut, they contend, would simply lead to a ''radicalization'' of the Palestinian nationalist movement and ultimately to the discrediting of the moderate Arab regimes which have supported the PLO's mainstream faction, al Fatah.
Saudi Arabia's King Fahd has been described as playing a more active role in behind-the-scenes mediation in this crisis than any previous Saudi King had played in other Middle East crises. Fahd has been in frequent contact by telephone with the PLO chairman, Yasser Arafat. The Saudis are said to be the chief financial backers of al Fatah.
One diplomat described the position of Saudi, French, and PLO officials on Lebanon as close, but not identical. The same diplomat said there was still ''a certain distance'' between the US and French positions and the US and Saudi positions, particularly when it came to the possibility of future negotiations over the Palestinian issue.
At his breakfast meeting with reporters, meanwhile, Defense Secretary Weinberger indicated that France might be a part of an evacuation force used to remove the PLO fighting forces from Lebanon. The French Foreign Ministry issued a statement July 6 saying that France was considering the possibility of participating in an international peace-keeping force for Lebanon that could come under the United Nations or which could consist of French or American troops or both.
The French and Saudis argue that if the PLO agrees to operate in the future only as a political organization and not as a military force, the Israelis and Americans should then drop their opposition to direct negotiations with the PLO.